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

[Erich Keane via cfe-commits]

erichkeane wrote:

As background: That chunking behavior was not in the specification document that I was provided internally when I implemented that.  IIRC at the time, regcall wasn't actually documented externally, so that documentation likely post-dates when I did my implementation.

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