c++, coroutines: Improve diagnostics for awaiter/promise.

c++, coroutines: Improve diagnostics for awaiter/promise.

At present, we can issue diagnostics about missing or malformed
awaiter or promise methods when we encounter their uses in the
body of a user's function.  We might then re-issue the same
diagnostics when processing the initial or final await expressions.

This change avoids such duplication, and also attempts to
identify issues with the initial or final expressions specifically
since diagnostics for those do not have any useful line number.

gcc/cp/ChangeLog:

	* coroutines.cc (build_co_await): Identify diagnostics
	for initial and final await expressions.
	(cp_coroutine_transform::wrap_original_function_body): Do
	not handle initial and final await expressions here ...
	(cp_coroutine_transform::apply_transforms): ... handle them
	here and avoid duplicate diagnostics.
	* coroutines.h: Declare inital and final await expressions
	in the transform class.  Save the function closing brace
	location.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/coro1-missing-await-method.C: Adjust for
	improved diagnostics.
	* g++.dg/coroutines/coro-missing-final-suspend.C: Likewise.
	* g++.dg/coroutines/pr104051.C: Move to...
	* g++.dg/coroutines/pr104051-0.C: ...here.
	* g++.dg/coroutines/pr104051-1.C: New test.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>