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

[Matt Arsenault via cfe-commits]

https://github.com/arsenm approved this pull request.

There are definitely places that would benefit from a getDeclaration. There are several places that have to awkwardly construct the intrinsic name to check getFunction 

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