[clang-linker-wrapper] Re-use type returned from 'PointerType::getUnqual(C)' (NFC) (#73374)

Multiple calls to `PointerType::getUnqual(C)`, and calls to
`Type::getPointerTo(AddrSpace=0)` on them all result in the same type.

Clean them up to re-use the same `PtrTy` variable within function
`createRegisterGlobalsFunction()`.