[CSSPGO][llvm-profgen] Handle return to external transition.

In a callback case, a return from internal code, say A, to external runtime can happen. The external runtime can then call back to another internal routine, say B. Making an artificial branch that looks like a return from A to B can confuse the unwinder to treat the instruction before B as the call instruction.

Reviewed By: wenlei, wmi

Differential Revision: https://reviews.llvm.org/D104546