[-cxx-abi microsoft] Mangle UUIDs correctly, stick them in the proper section

We mangled them like:
L___uuid_12345678-1234-1234-1234-123456789abc

We should've mangled them like:
__GUID_12345678_1234_1234_1234_123456789abc

Furthermore, they are external symbols.

llvm-svn: 188053