c++: Fix two issues with auto function parameter [PR99806]

c++: Fix two issues with auto function parameter [PR99806]

When we have a member function with auto parameter like this:

  struct S {
    void f(auto);
  };

cp_parser_member_declaration -> grokfield produces a FUNCTION_DECL
"void S::foo(auto:1)", and then finish_fully_implicit_template turns
that FUNCTION_DECL into a TEMPLATE_DECL.  The bug here is that we only
call cp_parser_save_default_args for a FUNCTION_DECL.  As a consequence,
abbrev10.C is rejected because we complain that the default argument has
not been defined, and abbrev11.C ICEs, because we don't re-parse the
delayed noexcept, so the DEFERRED_PARSE tree leaks into tsubst* where we
crash.  This patch fixes both issues.

gcc/cp/ChangeLog:

	PR c++/99806
	* parser.c (cp_parser_member_declaration): Call
	cp_parser_save_default_args even for function templates.  Use
	STRIP_TEMPLATE on the declaration we're passing.

gcc/testsuite/ChangeLog:

	PR c++/99806
	* g++.dg/concepts/abbrev10.C: New test.
	* g++.dg/concepts/abbrev11.C: New test.