[cmake] Remove obsolete files, docs and CMake variables related to the standalone build (#112741)

The runtimes used to support a build mode called the "Standalone build",
which isn't supported anymore (and hasn't been for a few years).
However, various places in the code still contained stuff whose only
purpose was to support that build mode, and some outdated documentation.
This patch cleans that up (although I probably missed some).

- Remove HandleOutOfTreeLLVM.cmake which isn't referenced anymore
- Remove the LLVM_PATH CMake variable which isn't used anymore
- Update some outdated documentation referencing standalone builds

Author: ldionne

cmake/Modules/HandleOutOfTreeLLVM.cmake

@@ -690,7 +690,6 @@ macro(add_custom_libcxx name prefix)
                -DCMAKE_CXX_FLAGS=${LIBCXX_CXX_FLAGS}
                -DCMAKE_BUILD_TYPE=Release
                -DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY
-               -DLLVM_PATH=${LLVM_MAIN_SRC_DIR}
                -DLLVM_ENABLE_RUNTIMES=libcxx|libcxxabi
                -DLIBCXXABI_USE_LLVM_UNWINDER=OFF
                -DLIBCXXABI_ENABLE_SHARED=OFF

compiler-rt/cmake/Modules/AddCompilerRT.cmake

@@ -16,7 +16,7 @@ On Mac OS, the easiest way to get this library is to link with -lSystem.
 However if you want to build tip-of-trunk from here (getting the bleeding
 edge), read on.
 
-The basic steps needed to build libc++ are:
+The basic steps needed to build libunwind are:
 
 #. Checkout LLVM, libunwind, and related projects:
 
@@ -50,23 +50,6 @@ The basic steps needed to build libc++ are:
    * ``make install-unwind`` --- Will install the libraries and the headers
 
 
-It is sometimes beneficial to build outside of the LLVM tree. An out-of-tree
-build would look like this:
-
-.. code-block:: bash
-
-  $ cd where-you-want-libunwind-to-live
-  $ # Check out llvm, and libunwind
-  $ ``svn co https://llvm.org/svn/llvm-project/llvm/trunk llvm``
-  $ ``svn co https://llvm.org/svn/llvm-project/libunwind/trunk libunwind``
-  $ cd where-you-want-to-build
-  $ mkdir build && cd build
-  $ export CC=clang CXX=clang++
-  $ cmake -DLLVM_PATH=path/to/llvm \
-          path/to/libunwind
-  $ make
-
-
 .. _CMake Options:
 
 CMake Options

libunwind/docs/BuildingLibunwind.rst

@@ -73,19 +73,11 @@ The procedure is:
 
 It is also possible to cross-compile from Linux.
 
-One method of building the libraries in step 2. is to build them "stand-alone".
-A stand-alone build doesn't involve the rest of the LLVM tree. The steps are:
-
-* ``cd build-dir``
-* ``cmake -DLLVM_PATH=<path to llvm checkout e.g. /llvm-project/> -DCMAKE_INSTALL_PREFIX=<install path> <other options> <path to project e.g. /llvm-project/libcxxabi>``
-* ``<make program e.g. ninja>``
-* ``<make program> install``
-
-More information on standalone builds can be found in the build documentation for
-the respective libraries. The next section discuss the salient options and modifications
-required for building and installing the libraries using standalone builds. This assumes
-that we are building libunwind and ibc++ as DLLs and statically linking libc++abi into
-libc++. Other build configurations are possible, but they are not discussed here.
+To build the libraries in step 2, refer to the `libc++ documentation <https://libcxx.llvm.org/VendorDocumentation.html#the-default-build>`_.
+
+The next section discuss the salient options and modifications required for building and installing the
+libraries. This assumes that we are building libunwind and libc++ as DLLs and statically linking libc++abi
+into libc++. Other build configurations are possible, but they are not discussed here.
 
 Common CMake configuration options:
 -----------------------------------

llvm/docs/HowToBuildWindowsItaniumPrograms.rst

@@ -75,9 +75,6 @@ set(LLVM_RUNTIME_OUTPUT_INTDIR ${LLVM_TOOLS_BINARY_DIR})
 set(LLVM_MAIN_SRC_DIR ${CMAKE_CURRENT_SOURCE_DIR}/../llvm)
 set(LLVM_CMAKE_DIR ${LLVM_MAIN_SRC_DIR}/cmake/modules)
 
-# This variable is used by individual runtimes to locate LLVM files.
-set(LLVM_PATH ${CMAKE_CURRENT_SOURCE_DIR}/../llvm)
-
 include(CheckLibraryExists)
 include(LLVMCheckCompilerLinkerFlag)
 include(CheckCCompilerFlag)