Avoid double substitution with complete explicit template arguments.

Avoid double substitution with complete explicit template arguments.

Previously, when we got a function template with explicit arguments for all
of the template parameters, we still did "deduction", which of course
couldn't deduce anything, but did other deduction-time checking of
non-dependent conversions and such.  This broke down with the unevaluated
lambdas patch (to follow): substituting into the lambda multiple times, once
to get the function type for deduction and then again to generate the actual
decl, doesn't work, since different substitutions of a lambda produce
different types.  I believe that skipping the initial substitution when we
have all the arguments is still conformant, and produces better diagnostics
for some testcases.

	* pt.c (fn_type_unification): If we have a full set of explicit
	arguments, go straight to substitution.

From-SVN: r266055