c++: concepts and conversions, take 2 [PR122127]

c++: concepts and conversions, take 2 [PR122127]

My r16-4115 changed convert_template_argument (when called from
instantiate_alias_template) to take the maybe_convert_nontype_argument path
rather than convert_nontype_argument for this testcase.  This meant not
folding the use of the by-ref capture in the template argument to constant
1.  When we come back to convert_template_argument again when substituting
into resize<_Np> we strip the IMPLICIT_CONV_EXPR (since the types are the
same) and take the convert_nontype_argument path, but at this point we've
pushed into the alias template context and trying to fold away the capture
fails because current_lambda_expr() is now null.

Taking the convert_nontype_argument path in the same-type case where we
would later strip an IMPLICIT_CONV_EXPR fixes the problem.  Note that
maybe_convert_nontype_argument already shares the same-type check with the
stripping, but that isn't enough; it still doesn't fold away the capture
proxy.

	PR c++/122127
	PR c++/112632

gcc/cp/ChangeLog:

	* pt.cc (convert_template_argument): Don't add redundant
	IMPLICIT_CONV_EXPR.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/lambda/lambda-template18.C: New test.