c++: Fix pragma suppression of -Wc++20-compat diagnostics [PR106423]

Gcc's '#pragma GCC diagnostic' directives are processed in "early mode"
(see handle_pragma_diagnostic_early) for the C++ frontend and, as such,
require that the target diagnostic option be enabled for the preprocessor
(see c_option_is_from_cpp_diagnostics).  This change modifies the
-Wc++20-compat option definition to register it as a preprocessor option
so that its associated diagnostics can be suppressed.  The changes also
implicitly disable the option in C++20 and later modes.  These changes
are consistent with the definition of the -Wc++11-compat option.

This support is motivated by the need to suppress the following diagnostic
otherwise issued in C++17 and earlier modes due to the char8_t typedef
present in the uchar.h header file in glibc 2.36.
  warning: identifier ‘char8_t’ is a keyword in C++20 [-Wc++20-compat]

Tests are added to validate suppression of both -Wc++11-compat and
-Wc++20-compat related diagnostics (fixes were only needed for the C++20
case).

	PR c++/106423

gcc/c-family/ChangeLog:
	* c-opts.cc (c_common_post_options): Disable -Wc++20-compat
	diagnostics in C++20 and later.
	* c.opt (Wc++20-compat): Enable hooks for the preprocessor.

gcc/cp/ChangeLog:
	* parser.cc (cp_lexer_saving_tokens): Add comment regarding
	diagnostic requirements.

gcc/testsuite/ChangeLog:
	* g++.dg/cpp0x/keywords2.C: New test.
	* g++.dg/cpp2a/keywords2.C: New test.

libcpp/ChangeLog:
	* include/cpplib.h (cpp_warning_reason): Add CPP_W_CXX20_COMPAT.
	* init.cc (cpp_create_reader): Add cpp_warn_cxx20_compat.