Merge commit 4ca2cad94 from llvm git (by Justin Hibbits):

Merge commit 4ca2cad94 from llvm git (by Justin Hibbits):

  [PowerPC] Add clang -msvr4-struct-return for 32-bit ELF

  Summary:

  Change the default ABI to be compatible with GCC. For 32-bit ELF
  targets other than Linux, Clang now returns small structs in
  registers r3/r4. This affects FreeBSD, NetBSD, OpenBSD. There is no
  change for 32-bit Linux, where Clang continues to return all structs
  in memory.

  Add clang options -maix-struct-return (to return structs in memory)
  and -msvr4-struct-return (to return structs in registers) to be
  compatible with gcc. These options are only for PPC32; reject them on
  PPC64 and other targets. The options are like -fpcc-struct-return and
  -freg-struct-return for X86_32, and use similar code.

  To actually return a struct in registers, coerce it to an integer of
  the same size. LLVM may optimize the code to remove unnecessary
  accesses to memory, and will return i32 in r3 or i64 in r3:r4.

  Fixes PR#40736

  Patch by George Koehler!

  Reviewed By: jhibbits, nemanjai
  Differential Revision: https://reviews.llvm.org/D73290

Requested by:	jhibbits
MFC after:	3 days