[NFC] Rename `Intrinsic::getDeclaration` to `getOrInsertDeclaration` (#111752)

Rename the function to reflect its correct behavior and to be consistent
with `Module::getOrInsertFunction`. This is also in preparation of
adding a new `Intrinsic::getDeclaration` that will have behavior similar
to `Module::getFunction` (i.e, just lookup, no creation).

GitOrigin-RevId: fa789dffb1e12c2aece0187aeacc48dfb1768340
2 files changed