Revert "[InstrProf][compiler-rt] Enable MC/DC Support in LLVM Source-based Code Coverage (1/3)"

This seems to cause Clang to crash, see comments on the code review. Reverting
until the problem can be investigated.

> Part 1 of 3. This includes the LLVM back-end processing and profile
> reading/writing components. compiler-rt changes are included.
>
> Differential Revision: https://reviews.llvm.org/D138846

This reverts commit a50486fd.