MS ABI: Mangle lambdas

Use a scheme inspired by the Itanium ABI to properly implement the
mangling of lambdas.

N.B.  The incredibly astute observer will notice that we do not generate
external names that are identical, or even compatible with, MSVC.
This is fine because they don't generate names that they can use across
translation units.  Technically, we can generate any name we'd like so
long as that name wouldn't conflict with any other and would be stable
across translation units.

This fixes PR15512.

llvm-svn: 202962