[Index] Make the USRs more stable.

- Only include offsets with local (in function scope) symbols, where we don't encode scoping
- Only include the filename with non-system symbols. Presumably the system headers will not provide conflicting definitions.

rdar://15976823

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201990 91177308-0d34-0410-b5e6-96231b3b80d8