[DFSan] Don't include public headers in dfsan runtime code

Summary:
Definitions we use in public sanitizer headers may
slightly conflict with the ones we use in private sanitizer runtimes.
Moreover, we generally forbid to include any system headers (like <stdint.h>)
in sanitizer runtime headers. This leads to inevitable duplication of selected
interface function declarations, but we decided to live with it.

Reviewers: pcc

Reviewed By: pcc

CC: kcc, llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2179