c++: Fix wrong-code with overloaded comma and CPP_EMBED [PR123737]

c++: Fix wrong-code with overloaded comma and CPP_EMBED [PR123737]

In cp_parser_expression for comma operator I've used a short path
where instead of calling build_x_compound_expr #embed number times
it is called just 3 times, for the CPP_NUMBER added by the preprocessor
at the start, last byte from CPP_EMBED and then CPP_NUMBER added by
libcpp at the end, enough to make sure -Wunused-value reports something,
but not bothering users with millions of -Wunused-value warnings
and spending too much compile time on it when they use a very large #embed.

As the following testcases show, that is ok for C or for C++ if the
expression before it is known not to have OVERLOAD_TYPE_P (common case
is INTEGER_TYPE I guess), but doesn't work well in case one uses overloaded
comma operator.  In that case we just have to call build_x_compound_expr
the right number of times, even if it is a lot.

I think I don't need to test for !expression, because the preprocessor
should guarantee that CPP_EMBED is preceded by CPP_NUMBER CPP_COMMA
tokens.

2026-01-24  Jakub Jelinek  <jakub@redhat.com>

	PR c++/123737
	* parser.cc (cp_parser_expression): Don't handle CPP_EMBED just
	as the last byte in it if expression has or might have overloaded
	type.  In that case call build_x_compound_expr for each byte
	in CPP_EMBED separately.

	* g++.dg/cpp/embed-28.C: New test.
	* g++.dg/parse/comma3.C: New test.