c++: warn about using keywords as identifiers [PR106111]

c++: warn about using keywords as identifiers [PR106111]

In C++03, -Wc++11-compat should warn about

  int constexpr;

since 'constexpr' is a keyword in C++11.  Jonathan reports that
we don't emit a similar warning for 'alignas' or 'alignof', and,
as I found out, 'thread_local'.

Similarly, we don't warn for most C++20 keywords.  That happens
because RID_LAST_CXX20 hasn't been updated in a while.

	PR c++/106111

gcc/c-family/ChangeLog:

	* c-common.h (enum rid): Update RID_LAST_CXX20.

gcc/cp/ChangeLog:

	* parser.cc (cp_lexer_get_preprocessor_token): Also warn about
	RID_ALIGNOF, RID_ALIGNAS, RID_THREAD.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/keywords1.C: New test.
	* g++.dg/cpp2a/keywords1.C: New test.