c++: typename in out-of-class member function definitions [PR97297]

I was notified that our P0634R3 (Down with typename) implementation has
a flaw: when we have an out-of-class member function definition, we
still required 'typename' for its parameters.  For example here:

  template <typename T> struct S {
    int simple(T::type);
  };
  template <typename T>
  int S<T>::simple(/* typename */T::type) { return 0; }

the 'typename' isn't necessary per [temp.res]/5.2.4.  We have a qualified
name here ("S<T>::simple") so we know it's already been declared so we
can look it up to see if it's a function template or a variable
template.

In this case, the P0634R3 code in cp_parser_direct_declarator wasn't
looking into uninstantiated templates and didn't find the member
function 'simple' -- cp_parser_lookup_name returned a SCOPE_REF which
means that the qualifying scope was dependent.  With this fix, we find
the BASELINK for 'simple', don't clear CP_PARSER_FLAGS_TYPENAME_OPTIONAL
from the flags, and the typename is implicitly assumed.

gcc/cp/ChangeLog:

	PR c++/97297
	* parser.c (cp_parser_direct_declarator): When checking if a
	name is a function template declaration for the P0634R3 case,
	look in uninstantiated templates too.

gcc/testsuite/ChangeLog:

	PR c++/97297
	* g++.dg/cpp2a/typename18.C: New test.