[libc++] Use c++20 instead of c++2a consistently.
* The only exception is that the flag -std=c++2a is still used not to break compatibility with older compilers (clang <= 9, gcc <= 9).
* Bump _LIBCPP_STD_VER for C++20 to 20 and use 21 for the future standard (C++2b).
That's a preparation step to add c++2b support to libc++.
Reviewed By: ldionne, #libc
Differential Revision: https://reviews.llvm.org/D93383
GitOrigin-RevId: 044b892c79b31a77bc0e6f52a0ec97c58b5b89b7
diff --git a/docs/DesignDocs/FeatureTestMacros.rst b/docs/DesignDocs/FeatureTestMacros.rst
index 644eb4a..2c6f983 100644
--- a/docs/DesignDocs/FeatureTestMacros.rst
+++ b/docs/DesignDocs/FeatureTestMacros.rst
@@ -8,7 +8,7 @@
Overview
========
-Libc++ implements the C++ feature test macros as specified in the C++2a standard,
+Libc++ implements the C++ feature test macros as specified in the C++20 standard,
and before that in non-normative guiding documents
(`See cppreference <https://en.cppreference.com/w/User:D41D8CD98F/feature_testing_macros>`_)