[clang][modules] NFC: Only sort interesting identifiers

In 9c254184 `ASTWriter` stopped writing identifiers that are not interesting. Taking it a bit further, we don't need to sort the whole identifier table, just the interesting identifiers. This reduces the size of sorted vector from ~10k (including lots of builtins) to 2 (`__VA_ARGS__` and `__VA_OPT__`) in a typical Xcode project, improving `clang-scan-deps` performance.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D150494