MS ABI: Preliminary RTTI mangling

Implement what we currently believe is the mangling scheme for RTTI
data.  Tests will be added in a later commit which actually generate
RTTI data.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208661 91177308-0d34-0410-b5e6-96231b3b80d8