libcpp: Support extended characters for #pragma {push,pop}_macro [PR109704]

The implementation of #pragma push_macro and #pragma pop_macro has to date
made use of an ad-hoc function, _cpp_lex_identifier(), which lexes an
identifier out of a string. When support was added for extended characters
in identifiers ($, UCNs, or UTF-8), that support was added only for the
"normal" way of lexing identifiers out of a cpp_buffer (_cpp_lex_direct) and
not for the ad-hoc way. Consequently, extended identifiers are not usable
with these pragmas.

The logic for lexing identifiers has become more complicated than it was
when _cpp_lex_identifier() was written -- it now handles things like \N{}
escapes in C++, for instance -- and it no longer seems practical to maintain
a redundant code path for lexing identifiers. Address the issue by changing
the implementation of #pragma {push,pop}_macro to lex identifiers in the
expected way, i.e. by pushing a cpp_buffer and lexing the identifier from
there.

The existing implementation has some quirks because of the ad-hoc parsing
logic. For example:

 #pragma push_macro("X ")
 ...
 #pragma pop_macro("X")

will not restore macro X (note the extra space in the first string). However:

 #pragma push_macro("X ")
 ...
 #pragma pop_macro("X ")

actually does sucessfully restore "X". This is because the key for looking
up the saved macro on the push stack is the original string passed, so the
string passed to pop_macro needs to match it exactly. It is not that easy to
reproduce this logic in the world of extended characters, given that for
example it should be valid to pass a UCN to push_macro, and the
corresponding UTF-8 to pop_macro. Given that this aspect of the existing
behavior seems unintentional and has no tests (and does not match other
implementations), I opted to make the new logic more straightforward. The
string passed needs to lex to one token, which must be a valid identifier,
or else no action is taken and no error is generated. Any diagnostics
encountered during lexing (e.g., due to a UTF-8 character not permitted to
appear in an identifier) are also suppressed.

It could be nice (for GCC 15) to also add a warning if a pop_macro does not
match a previous push_macro.

libcpp/ChangeLog:

	PR preprocessor/109704
	* include/cpplib.h (class cpp_auto_suppress_diagnostics): New class.
	* errors.cc
	(cpp_auto_suppress_diagnostics::cpp_auto_suppress_diagnostics): New
	function.
	(cpp_auto_suppress_diagnostics::~cpp_auto_suppress_diagnostics): New
	function.
	* charset.cc (noop_diagnostic_cb): Remove.
	(cpp_interpret_string_ranges): Refactor diagnostic suppression logic
	into new class cpp_auto_suppress_diagnostics.
	(count_source_chars): Likewise.
	* directives.cc (cpp_pop_definition): Add cpp_hashnode argument.
	(lex_identifier_from_string): New static helper function.
	(push_pop_macro_common): Refactor common logic from
	do_pragma_push_macro and do_pragma_pop_macro; use
	lex_identifier_from_string instead of _cpp_lex_identifier.
	(do_pragma_push_macro): Reimplement using push_pop_macro_common.
	(do_pragma_pop_macro): Likewise.
	* internal.h (_cpp_lex_identifier): Remove.
	* lex.cc (lex_identifier_intern): Remove.
	(_cpp_lex_identifier): Remove.

gcc/testsuite/ChangeLog:

	PR preprocessor/109704
	* c-c++-common/cpp/pragma-push-pop-utf8.c: New test.
	* g++.dg/pch/pushpop-2.C: New test.
	* g++.dg/pch/pushpop-2.Hs: New test.
	* gcc.dg/pch/pushpop-2.c: New test.
	* gcc.dg/pch/pushpop-2.hs: New test.