LLVM: fix C ABI for windows

 * sret logic needed a check for hasRuntimeBits()
 * lower f128 on windows targets with the "sse" class rather than
   "memory". For reference, clang emits a compile error when __float128
   is used with the MSVC ABI, saying that this type is not supported.
   The docs for the x64 calling convention have both of these sentences:
   - "Any argument that doesn't fit in 8 bytes, or isn't 1, 2, 4, or 8 bytes,
     must be passed by reference."
   - "All floating point operations are done using the 16 XMM registers."
 * For i128, however, it is clear that the Windows calling convention
   wants such an object to be passed by reference. I fixed the LLVM
   lowering for function parameters to make this work.

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