WIP implementation of explicit function template specialization. This

first implementation recognizes when a function declaration is an
explicit function template specialization (based on the presence of a
template<> header), performs template argument deduction + ambiguity
resolution to determine which template is being specialized, and hooks

There are many caveats here:
  - We completely and totally drop any explicitly-specified template
  arguments on the floor
  - We don't diagnose any of the extra semantic things that we should
  diagnose. 
  - I haven't looked to see that we're getting the right linkage for
  explicit specializations

On a happy note, this silences a bunch of errors that show up in
libstdc++'s <iostream>, although Clang still can't get through the
entire header.

llvm-svn: 82728