c++: premature requires-expression folding [PR95020]

c++: premature requires-expression folding [PR95020]

In the testcase below we're prematurely folding away the
requires-expression to 'true' after substituting in the function's
template arguments, but before substituting in the lambda's deduced
template arguments.

This patch removes the uses_template_parms check when deciding in
tsubst_requires_expr whether to keep around a new requires-expression.
Regardless of whether the template arguments are dependent, there still
might be more template parameters to later substitute in (as in the
below testcase) and even if not, tsubst_expr doesn't perform full
semantic processing unless !processing_template_decl, so we should still
wait until then to fold away the requires-expression.

gcc/cp/ChangeLog:

	PR c++/95020
	* constraint.cc (tsubst_requires_expr): Produce a new
	requires-expression when processing_template_decl, even if
	template arguments are not dependent.

gcc/testsuite/ChangeLog:

	PR c++/95020
	* g++.dg/cpp2a/concepts-lambda7.C: New test.

(cherry picked from commit 7e52f8b1e03776575b92574252d9b6bbed9f1af4)