[demangler] NFC: Some refactoring to support partial demangling.

I'm committing this to libcxxabi too so that the two demanglers remain as
simular as possible.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@329950 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed