[clang] [C23] Select the correct promoted type for a bit-field (PR #89254)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

@tbaederr -- this triggers an assertion with the new constexpr interpreter in `clang/test/AST/Interp/intap.cpp` but I'm not certain I understand why. Can you help me figure out how to fix that?

https://github.com/llvm/llvm-project/pull/89254