[SPARC] Make calls to function with big return values work

Implement CanLowerReturn and associated CallingConv changes for SPARC/SPARC64.

In particular, for SPARC64 there's new `RetCC_Sparc64_*` functions that handles the return case of the calling convention.
It uses the same analysis as `CC_Sparc64_*` family of funtions, but fails if the return value doesn't fit into the return registers.

This makes calls to functions with big return values converted to an sret function as expected, instead of crashing LLVM.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D132465