Initial support for struct-path aware TBAA.

Added TBAABaseType and TBAAOffset in LValue. These two fields are initialized to
the actual type and 0, and are updated in EmitLValueForField.
Path-aware TBAA tags are enabled for EmitLoadOfScalar and EmitStoreOfScalar.
Added command line option -struct-path-tbaa.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178797 91177308-0d34-0410-b5e6-96231b3b80d8