Use BuildReturnStmt in SemaCoroutine to unbreak sanitizer tests.

FIXME: ActOnReturnStmt expects a scope that is inside of the function, due
  to CheckJumpOutOfSEHFinally(*this, ReturnLoc, *CurScope->getFnParent());
  S.getCurScope()->getFnParent() == nullptr at ActOnFinishFunctionBody when
  CoroutineBodyStmt is built. Figure it out and fix it.
llvm-svn: 298893