Fix thunks returning memptrs via sret by emitting also scalar return values...

Fix thunks returning memptrs via sret by emitting also scalar return values directly in sret slot (PR39901)

Thunks that return member pointers via sret are broken due to using temporary
storage for the return value on the stack and then passing that pointer to a
tail call, violating the rule that a tail call can't access allocas in the
caller (see bug).

Since r90526, we put aggregate return values directly in the sret slot, but
this doesn't apply to member pointers which are considered scalar.

Unless I'm missing something subtle, we should be able to always use the sret
slot directly for indirect return values.

Differential revision: https://reviews.llvm.org/D55371

llvm-svn: 348569