[XCore] Make LowerCallResult a static function.

No functionality change. This is r202396 reapplied with no changes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202413 91177308-0d34-0410-b5e6-96231b3b80d8
[XCore] Support functions returning more than 4 words.

If a function returns a large struct by value return the first 4 words
in registers and the rest on the stack in a location reserved by the
caller. This is needed to support the xC language which supports
functions returning an arbitrary number of return values. This is
r202397 reapplied with a fix to avoid an uninitialized read of a member.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202414 91177308-0d34-0410-b5e6-96231b3b80d8