[msan] Add debugging for handleUnknownIntrinsic (#123381)
This adds an experimental flag, msan-dump-strict-intrinsics (modeled
after msan-dump-strict-instructions), which prints out any intrinsics
that are heuristically handled. Additionally, MSan will print out
heuristically handled intrinsics when -debug is passed as a flag in
debug builds.
MSan's intrinsic handling can be broken down into:
1) special cases (usually highly accurate)
2) heuristic handling (sometimes erroneous)
3) not handled
This patch's -msan-dump-strict-intrinsics is intended to help debug Case
2. Case 3) (which includes all the heuristics that are not handled by
special cases nor heuristics) can be debugged using the existing
-msan-dump-strict-instructions.
