c++: ICE with 'this' in lambda signature [PR120748]

c++: ICE with 'this' in lambda signature [PR120748]

This testcase was crashing from infinite recursion in the diagnostic
machinery, trying to print the lambda signature, which referred to the
__this capture field in the lambda, which wanted to print the lambda again.

But we don't want the signature to refer to the capture field; 'this' in an
unevaluated context refers to the 'this' from the enclosing function, not
the capture.

After fixing that, we still wrongly rejected the B case because
THIS_FORBIDDEN is set in a default (template) argument.  Since we don't
distinguish between THIS_FORBIDDEN being set for a default argument and it
being set for a static member function, let's just ignore it if
cp_unevaluated_operand; we'll give a better diagnostic for the static memfn
case in finish_this_expr.

	PR c++/120748

gcc/cp/ChangeLog:

	* lambda.cc (lambda_expr_this_capture): Don't return a FIELD_DECL.
	* parser.cc (cp_parser_primary_expression): Ignore THIS_FORBIDDEN
	if cp_unevaluated_operand.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-targ16.C: New test.
	* g++.dg/cpp0x/this1.C: Adjust diagnostics.