MS ABI: Support mangling of return-types deducing to local types

The MS ABI forces us into catch-22 when it comes to functions which
return types which are local:

 - A function is mangled with it's return type.
 - A type is mangled with it's surrounding context.

Avoid this by mangling auto and decltype(autp) directly into the
function's return type.  Using this mangling has the double advantage of
being compatible with the C++ standard without crashing the compiler.

N.B. For the curious, the MSVC mangling leads to collisions amongst
template functions and either crashes when faced with local types or is
otherwise incapable of returning them.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205282 91177308-0d34-0410-b5e6-96231b3b80d8