[clang] added regcall struct by reg support (PR #95257)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Right, the specification requires splitting the whole structure into chunks; if we add a special-case for 8-byte structs, we'll just have to throw it away when we implement the right algorithm.

Also, I'm not sure what the isBuiltinType() check is supposed to handle.  It seems to exclude values you'd want to handle, like pointers.

https://github.com/llvm/llvm-project/pull/95257