c++: Check invalid use of constrained auto with trailing return type [PR100589]

c++: Check invalid use of constrained auto with trailing return type [PR100589]

Add check for constrained auto type specifier in function declaration or
function type declaration with trailing return type. Issue error if such
usage is detected.

Test file renamed, and added a new test for type declaration.

Successfully bootstrapped and regretested on x86_64-pc-linux-gnu:
Added 6 passed and 4 unsupported tests.

	PR c++/100589

gcc/cp/ChangeLog:

	* decl.cc (grokdeclarator): Issue an error for a declarator with
	constrained auto type specifier and trailing return types. Include
	function names if available.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-pr100589.C: New test.

Signed-off-by: Da Xie <xxie_xd@163.com>
Reviewed-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jason Merrill <jason@redhat.com>