When checking the instantiation of a default template argument against

the template parameter, perform the checking as a "specified" template
argument rather than a "deduced" template argument; the latter implies
stricter type checking that is not permitted for default template
arguments.

Also, cleanup our handling of substitution of explicit template
arguments for a function template. We were actually performing some
substitution of default arguments at this point!

Fixes PR10069.

llvm-svn: 132529