[lldb][Docs] Document our major differences from the LLVM style (#66345)

Running:
$ clang-format -i $(find -regex "\./lldb/.*\.c") $(find -regex
"\./lldb/.*\.cpp") $(find -regex "\./lldb/.*\.h")

Resulted in:
 1602 files changed, 25090 insertions(+), 25849 deletions(-)
(note: this includes tests which we wouldn't format, just using this as
an example)

The vast majority of which were whitespace changes. So as far as
formatting we're not deviating from llvm for any reason other than not
churning old code.

Formatting aside, the major features of lldb (single line if, early
return) are all reflected in llvm's style. We differ mainly on variable
naming (proposed to change in
https://llvm.org/docs/Proposals/VariableNames.html anyway) and use of
asserts. Which was already documented.

Author: DavidSpickett

lldb/docs/resources/contributing.rst

@@ -18,19 +18,45 @@ Please refer to the `LLVM Developer Policy
 authoring and uploading a patch. LLDB differs from the LLVM Developer
 Policy in the following respects.
 
- - **Test infrastructure**: Like LLVM it is  important to submit tests with your
-   patches, but note that LLDB uses a different system for tests. Refer to the
-   `test documentation <test.html>`_ for more details and the ``lldb/test``
-   folder on disk for examples.
-
- - **Coding Style**: LLDB's code style differs from
-   `LLVM's coding style <https://llvm.org/docs/CodingStandards.html>`_.
-   Unfortunately there is no document describing the differences. Please be
-   consistent with the existing code.
-
 For anything not explicitly listed here, assume that LLDB follows the LLVM
 policy.
 
+Coding Style
+++++++++++++
+
+LLDB's code style differs from `LLVM's coding style <https://llvm.org/docs/CodingStandards.html>`_
+in a few ways. The 2 main ones are:
+
+* `Variable and function naming <https://llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly>`_:
+
+  * Variables are ``snake_case``.
+
+  * Functions and methods are ``UpperCamelCase``.
+
+  * Static, global and member variables have ``s_``, ``g_`` and ``_m``
+    prefixes respectively.
+
+* `Use of asserts <https://llvm.org/docs/CodingStandards.html#assert-liberally>`_:
+  See the :ref:`section below<Error Handling>`.
+
+For any other contradications, consider the
+`golden rule <https://llvm.org/docs/CodingStandards.html#introduction>`_
+before choosing to update the style of existing code.
+
+All new code in LLDB should be formatted with clang-format. Existing code may
+not conform and should be updated prior to being modified. Bulk reformatting
+is discouraged.
+
+Test Infrastructure
++++++++++++++++++++
+
+Like LLVM it is important to submit tests with your patches, but note that  a
+subset of LLDB tests (the API tests) use a different system. Refer to the
+`test documentation <test.html>`_ for more details and the
+`lldb/test <https://github.com/llvm/llvm-project/tree/main/lldb/test>`_ folder
+for examples.
+
+.. _Error handling:
 
 Error handling and use of assertions in LLDB
 --------------------------------------------
@@ -42,14 +68,13 @@ be extra thoughtful about how to handle errors. Below are a couple
 rules of thumb:
 
 * Invalid input.  To deal with invalid input, such as malformed DWARF,
-  missing object files, or otherwise inconsistent debug info, LLVM's
+  missing object files, or otherwise inconsistent debug info,
   error handling types such as `llvm::Expected<T>
   <https://llvm.org/doxygen/classllvm_1_1Expected.html>`_ or
-  `std::optional<T>
-  <https://llvm.org/doxygen/classllvm_1_1Optional.html>`_ should be
-  used. Functions that may fail should return their result using these
-  wrapper types instead of using a bool to indicate success. Returning
-  a default value when an error occurred is also discouraged.
+  ``std::optional<T>`` should be used. Functions that may fail
+  should return their result using these wrapper types instead of
+  using a bool to indicate success. Returning a default value when an
+  error occurred is also discouraged.
 
 * Assertions.  Assertions (from ``assert.h``) should be used liberally
   to assert internal consistency.  Assertions shall **never** be
@@ -71,16 +96,18 @@ rules of thumb:
   behaves like ``assert()``. When asserts are disabled, it will print a
   warning and encourage the user to file a bug report, similar to
   LLVM's crash handler, and then return execution. Use these sparingly
-  and only if error handling is not otherwise feasible.  Specifically,
-  new code should not be using ``lldbassert()`` and existing
-  uses should be replaced by other means of error handling.
+  and only if error handling is not otherwise feasible.
+
+.. note::
+
+  New code should not be using ``lldbassert()`` and existing uses should
+  be replaced by other means of error handling.
 
 * Fatal errors.  Aborting LLDB's process using
   ``llvm::report_fatal_error()`` or ``abort()`` should be avoided at all
-  costs.  It's acceptable to use `llvm_unreachable()
-  <https://llvm.org/doxygen/Support_2ErrorHandling_8h.html>`_ for
-  actually unreachable code such as the default in an otherwise
-  exhaustive switch statement.
+  costs.  It's acceptable to use ``llvm_unreachable()`` for actually
+  unreachable code such as the default in an otherwise exhaustive
+  switch statement.
 
 Overall, please keep in mind that the debugger is often used as a last
 resort, and a crash in the debugger is rarely appreciated by the