msan: Handle musttail calls

First, avoid calling setTailCall(false) on musttail calls.  The funciton
prototypes should be "congruent", so the shadow layout should be exactly
the same.

Second, avoid inserting instrumentation after a musttail call to
propagate the return value shadow.  We don't need to propagate the
result of a tail call, it should already be in the right place.

Reviewed By: eugenis

Differential Revision: http://reviews.llvm.org/D4331

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215415 91177308-0d34-0410-b5e6-96231b3b80d8