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

[Nikita Popov via llvm-commits]

nikic wrote:

TBH I am wondering if we should revert this rename. I don't think it's a good idea to reuse getDeclaration with substantially different semantics, and if we're not reusing it, then there's not much point to rename...

Instead of having getOrInsertDeclaration + getDeclaration with a meaning inversion, we can have getDeclaration and getDeclarationIfExists without the inversion.

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