BuiltinOperatorOverloadBuilder: Don't consider types that are unavailable on the target (PR35174)

In the PR, Clang ended up in a situation where it tried to mangle the
__float128 type, which isn't supported when targetingt MSVC, because
Clang instantiated a variable template with that type when searching for
a conversion to use in an arithmetic expression.

Differential revision: https://reviews.llvm.org/D39579

llvm-svn: 318309