[libc++] Add basic support for -std=c++2b.

* Add feature test macros.
* Add buildbot configuration generic-cxx2b that uses clang-tot.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D94227

GitOrigin-RevId: 95729f95d803c8e8f4ca3e6767404ec2c0492d53
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 377ea58..97edbb0 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -289,5 +289,15 @@
     ``__cpp_lib_to_array``                            ``201907L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_unwrap_ref``                          ``201811L``
+    ------------------------------------------------- -----------------
+    **C++ 2b**
+    -------------------------------------------------------------------
+    ``__cpp_lib_is_scoped_enum``                      *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_stacktrace``                          *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_stdatomic_h``                         *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_string_contains``                     *unimplemented*
     ================================================= =================