Fix dfsan handling of musttail calls.

Without this change, a callsite like:
  [[clang::musttail]] return func_call(x);
will cause an error like:
  fatal error: error in backend: failed to perform tail call elimination
  on a call site marked musttail
due to DFSan inserting instrumentation between the musttail call and
the return.

Reviewed By: stephan.yichao.zhao

Differential Revision: https://reviews.llvm.org/D103542