LLVM is gearing up to move to `std::endianness` and as part of that has
moved `llvm::support::endianness` to `llvm::endianness`
(bbdbcd83e6702f314d147a680247058a899ba261). Rename our uses.
LLVM is presumably moving towards `std::string_view` -
`StringRef::startswith` is deprecated on tip. `SmallString::startswith`
was just renamed there (maybe with some small deprecation inbetween, but
if so, we've missed it).
The `SmallString::startswith` references were moved to
`.str().starts_with()`, rather than adding the `starts_with` on
`stable/20230725` as we only had a few of them. Open to switching that
over if anyone feels strongly though.
