Add documentation for constexpr string builtin support.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@292558 91177308-0d34-0410-b5e6-96231b3b80d8

docs/LanguageExtensions.rst

@@ -1776,6 +1776,46 @@ numeric primitives such as frexp. See `LLVM canonicalize intrinsic
 <http://llvm.org/docs/LangRef.html#llvm-canonicalize-intrinsic>`_ for
 more information on the semantics.
 
+String builtins
+---------------
+
+Clang provides constant expression evaluation support for builtins forms of
+the following functions from the C standard library ``<strings.h>`` header:
+
+* ``memchr``
+* ``memcmp``
+* ``strchr``
+* ``strcmp``
+* ``strlen``
+* ``strncmp``
+* ``wcschr``
+* ``wcscmp``
+* ``wcslen``
+* ``wcsncmp``
+* ``wmemchr``
+* ``wmemcmp``
+
+In each case, the builtin form has the name of the C library function prefixed
+by ``__builtin``. Example:
+
+.. code-block:: c
+
+  void *p = __builtin_memchr("foobar", 'b', 5);
+
+In addition to the above, one further builtin is provided:
+
+.. code-block:: c
+
+  char *__builtin_char_memchr(const char *haystack, int needle, size_t size);
+
+``__builtin_char_memchr(a, b, c)`` is identical to
+``(char*)__builtin_memchr(a, b, c)`` except that its use is permitted within
+constant expressions in C++11 onwards (where a cast from ``void*`` to ``char*``
+is disallowed in general).
+
+Support for constant expression evaluation for the above builtins be detected
+with ``__has_feature(cxx_constexpr_string_builtins)``.
+
 .. _langext-__c11_atomic:
 
 __c11_atomic builtins