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

[Arthur Eubanks via cfe-commits]

aeubanks wrote:

for such a wide-reaching change, it would have been nice to add the new function in one commit, update users in other commits, then remove the old function in one last commit. makes it easier to revert the removal of the old function, and makes it easier for out of tree users to to migrate

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