[clang] [clang] Add per-global code model attribute (PR #72078)

[Reid Kleckner via cfe-commits]

rnk wrote:

Do folks feel like the attribute name is sufficiently descriptive? i.e. should it be `__attribute__((code_model("asdf")))`? Are we aiming for GCC compat here? What guides the naming choice?

https://github.com/llvm/llvm-project/pull/72078