MS ABI: Don't use the Itanium scheme for numbering lambdas

The Itanium ABI will give out the same mangling number for two different
lambdas if their call operators have different types.  The MS ABI cannot
do the same because it does not mangle the return type into it's
lambdas.

This fixes PR20719.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@216259 91177308-0d34-0410-b5e6-96231b3b80d8