c++: Fix P0846 (ADL and function templates) in template [PR97010]

c++: Fix P0846 (ADL and function templates) in template [PR97010]

To quickly recap, P0846 says that a name is also considered to refer to
a template if it is an unqualified-id followed by a < and name lookup
finds either one or more functions or finds nothing.

In a template, when parsing a function call that has type-dependent
arguments, we can't perform ADL right away so we set KOENIG_LOOKUP_P in
the call to remember to do it when instantiating the call
(tsubst_copy_and_build/CALL_EXPR).  When the called function is a
function template, we represent the call with a TEMPLATE_ID_EXPR;
usually the operand is an OVERLOAD.

In the P0846 case though, the operand can be an IDENTIFIER_NODE, when
name lookup found nothing when parsing the template name.  But we
weren't handling this correctly in tsubst_copy_and_build.  First
we need to pass the FUNCTION_P argument from <case TEMPLATE_ID_EXPR> to
<case IDENTIFIER_NODE>, otherwise we give a bogus error.  And then in
<case CALL_EXPR> we need to perform ADL.  The rest of the changes is to
give better errors when ADL didn't find anything.

gcc/cp/ChangeLog:

	PR c++/97010
	* pt.c (tsubst_copy_and_build) <case TEMPLATE_ID_EXPR>: Call
	tsubst_copy_and_build explicitly instead of using the RECUR macro.
	Handle a TEMPLATE_ID_EXPR with an IDENTIFIER_NODE as its operand.
	<case CALL_EXPR>: Perform ADL for a TEMPLATE_ID_EXPR with an
	IDENTIFIER_NODE as its operand.

gcc/testsuite/ChangeLog:

	PR c++/97010
	* g++.dg/cpp2a/fn-template21.C: New test.
	* g++.dg/cpp2a/fn-template22.C: New test.