MS ABI: Mangle this qualifiers on function types

C++11 allows this qualifiers to exist on function types when used in
template arguments.  Previously, I believed it wasn't possible because
MSVC rejected declarations like: S<int () const &> s;

However, it turns out MSVC properly allows them in using declarations;
updated clang to be compatible with this mangling.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@215464 91177308-0d34-0410-b5e6-96231b3b80d8