[clang][lex] Use `FileManager` to make prebuilt module paths absolute (#165347)
This PR switches from using `llvm::sys::fs::make_absolute()` to
`FileManager::makeAbsolutePath()` so that `FileSystemOptions` (i.e. the
`-working-directory` option) and the `VFS`'s CWD have a say in how the
prebuilt module paths are resolved. This matches how the rest of the
compiler treats input files.