[LangRef] Update initializes definition (llvm#134370)

Specify the initializes attribute in terms of an "initialized" shadow
state, such that:

* Loads prior to initialization return poison.
* Bytes that are not explicitly initialized are written with undef on
function return.

This is intended to preserve the core semantics of the attribute, but
adjusts the wording in a way that is compatible with existing
optimizations, such as insertion of spurious loads and removal of
uninitialized writes.

Fixes llvm#133038.
Fixes llvm#133059.

Author: nikic

llvm/docs/LangRef.rst

@@ -1690,10 +1690,24 @@ Currently, only the following parameter attributes are defined:
 
 ``initializes((Lo1, Hi1), ...)``
     This attribute indicates that the function initializes the ranges of the
-    pointer parameter's memory, ``[%p+LoN, %p+HiN)``. Initialization of memory
-    means the first memory access is a non-volatile, non-atomic write. The
-    write must happen before the function returns. If the function unwinds,
-    the write may not happen.
+    pointer parameter's memory ``[%p+LoN, %p+HiN)``. Colloquially, this means
+    that all bytes in the specified range are written before the function
+    returns, and not read prior to the initializing write. If the function
+    unwinds, the write may not happen.
+
+    Formally, this is specified in terms of an "initialized" shadow state for
+    all bytes in the range, which is set to "not initialized" at function entry.
+    If a memory access is performed through a pointer based on the argument,
+    and an accessed byte has not been marked as "initialized" yet, then:
+
+     * If the byte is stored with a non-volatile, non-atomic write, mark it as
+       "initialized".
+     * If the byte is stored with a volatile or atomic write, the behavior is
+       undefined.
+     * If the byte is loaded, return a poison value.
+
+    Additionally, if the function returns normally, write an undef value to all
+    bytes that are part of the range and have not been marked as "initialized".
 
     This attribute only holds for the memory accessed via this pointer
     parameter. Other arbitrary accesses to the same memory via other pointers