[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/UsingLibcxx.rst b/docs/UsingLibcxx.rst
index 4c37ada..6bf67ce 100644
--- a/docs/UsingLibcxx.rst
+++ b/docs/UsingLibcxx.rst
@@ -254,7 +254,7 @@
**_LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR**:
This macro is used to re-enable `std::auto_ptr` in C++17.
-C++2a Specific Configuration Macros:
+C++20 Specific Configuration Macros:
------------------------------------
**_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17**:
This macro can be used to disable diagnostics emitted from functions marked