[clang] [clang] Fix crashes when passing VLA to va_arg (PR #119563)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

C standard rules for va_arg: "[...] if *type* is not compatible with the type of the actual next argument [...], the behavior is undefined [...]".  A struct is never compatible with an array, so yes , it's undefined.  (See 6.2.7 for what constitutes a "compatible type".)

As a practical matter, breaking the "compatible type" rule can cause crashes or data corruption.  Modern ABIs don't just pass everything on the stack.

https://github.com/llvm/llvm-project/pull/119563