Reuse the SelectorTable from Clang's Preprocessor

Summary:
At the moment we create our own SelectorTable even though the Preprocessor always
creates one for us that we can (and should) reuse.

Reviewers: vsk

Reviewed By: vsk

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D51185

llvm-svn: 340585