Start implementing the Microsoft-style name mangler. Mangle simple names

(but not their types; that's later).

NOTE: Right now, variables in the global namespace don't get mangled, even
though they're supposed to be. This is because the default mangler
implements the shouldMangleDeclName() method that tells clang not to mangle
them. This will be fixed in a later patch.

llvm-svn: 105805