MS ABI: Mangle lambdas which are given the same mangling number

It is possible for lambdas to get the same mangling number because they
may exist in different mangling contexts.  To handle this correctly,
mangle the context into the name as well.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214947 91177308-0d34-0410-b5e6-96231b3b80d8