[clang] [llvm] [mlir] [polly] [NFC] Rename `Intrinsic::getDeclaration` to `getOrInsertDeclaration` (PR #111752)

[Nikita Popov via cfe-commits]

nikic wrote:

To reuse the name with different semantics, I'd put removal and reuse in different releases, so it takes about half a year. (Deprecation itself can be very short-lived.)

https://github.com/llvm/llvm-project/pull/111752