[TableGen] Refactor TableGenParseFile to no longer use a callback

Now that TableGen no longer relies on global Record state, we can allow
for the client to own the RecordKeeper and SourceMgr. Given that TableGen
internally still relies on the global llvm::SrcMgr, this method unfortunately
still isn't thread-safe.

Differential Revision: https://reviews.llvm.org/D125277