[clang] [CodeGen][NFCI] Don't re-implement parts of ASTContext::getIntWidth (PR #101765)

[Jessica Clarke via cfe-commits]

jrtc27 wrote:

For the avoidance of doubt, I would appreciate a review from a core Clang developer, given Alex also comes from a CHERI LLVM background.

https://github.com/llvm/llvm-project/pull/101765