DAG: move sret demotion into most basic LowerCallTo implementation.

It looks like there are two versions of LowerCallTo here: the
SelectionDAGBuilder one is designed to operate on LLVM IR, and the
TargetLowering one in the case where everything is at DAG level.

Previously, only the SelectionDAGBuilder variant could handle
demoting an impossible return to sret semantics (before delegating to
the TargetLowering version), but this functionality is also useful
for certain libcalls (e.g. 128-bit operations on 32-bit x86).  So
this commit moves the sret handling down a level.

rdar://problem/17242889