[clang] [clang] Function type attribute to prevent CFI instrumentation (PR #135836)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I'm confident making this part of the FunctionType is the right approach.

We don't implicitly drop the bits from the FunctionType bitfields, for bits that are part of the canonical type.  Any cast is explicitly represented in the AST, and we have warnings for mismatches (see checkCastFunctionType()).  The warnings are just off by default because weird casts are common in practice.

https://github.com/llvm/llvm-project/pull/135836