Turn struct-path aware TBAA on by default.

Use -no-struct-path-tbaa to turn it off.
This is the same as r191695, which was reverted because it depends on a
commit that has issues.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192497 91177308-0d34-0410-b5e6-96231b3b80d8