MS ABI: Use '1' (instead of '0') relative scope discriminators

This changes the scope discriminator's behavior to start at '1' instead
of '0'.  Symbol table diffing, for ABI compatibility testing, kept
finding these as false positives.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219075 91177308-0d34-0410-b5e6-96231b3b80d8