[CGData] Stop exporting cl::opts. NFC (#226861)
LTO reads `-codegen-data-thinlto-two-rounds` through
`extern cl::opt<bool> CodeGenDataThinLTOTwoRounds`, and llvm-cgdata
assigns
`IndexedCodeGenDataLazyLoading`, exported from CodeGenDataReader.h. Add
`cgdata::thinLTOTwoRounds()` for LTO, and pass lazy loading to
`CodeGenDataReader::create` as a parameter, so that the options can be
file-local.
Aided by Opus 5.5
