[Libcxx] Add <source_location> header.

This requires the __builtin_source_location() builtin, as implemented
by GCC and Clang.

Fixes https://github.com/llvm/llvm-project/issues/56363

Differential Revision: https://reviews.llvm.org/D120634

NOKEYCHECK=True
GitOrigin-RevId: 73d94b19161355d06f20678d628555719eebbdcc
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 34e28a3..a71e1e6 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -274,7 +274,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_smart_ptr_for_overwrite``             *unimplemented*
     ------------------------------------------------- -----------------
-    ``__cpp_lib_source_location``                     *unimplemented*
+    ``__cpp_lib_source_location``                     ``201907L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_span``                                ``202002L``
     ------------------------------------------------- -----------------