c++: Fix up ICEs on constexpr inline asm strings in templates [PR118277]

c++: Fix up ICEs on constexpr inline asm strings in templates [PR118277]

The following patch fixes ICEs when the new inline asm syntax
to use C++26 static_assert-like constant expressions in place
of string literals is used in templates.
As finish_asm_stmt doesn't do any checking for
processing_template_decl, this patch also just defers handling
those strings in templates rather than say trying fold_non_dependent_expr
and if the result is non-dependent and usable, try to extract.

The patch also reverts changes to cp_parser_asm_specification_opt
which allowed something like
void foo () asm ((std::string_view ("bar")));
but it would be really hard to support
template <int N>
void baz () asm ((std::string_view ("qux")));
(especially with dependent constant expression).

And the patch adds extensive test coverage for the various errors.

2025-01-10  Jakub Jelinek  <jakub@redhat.com>

	PR c++/118277
	* cp-tree.h (finish_asm_string_expression): Declare.
	* semantics.cc (finish_asm_string_expression): New function.
	(finish_asm_stmt): Use it.
	* parser.cc (cp_parser_asm_string_expression): Likewise.
	Wrap string into PAREN_EXPR in the ("") case.
	(cp_parser_asm_definition): Don't ICE if finish_asm_stmt
	returns error_mark_node.
	(cp_parser_asm_specification_opt): Revert 2024-06-24 changes.
	* pt.cc (tsubst_stmt): Don't ICE if finish_asm_stmt returns
	error_mark_node.

	* g++.dg/cpp1z/constexpr-asm-4.C: New test.
	* g++.dg/cpp1z/constexpr-asm-5.C: New test.