[libclang] Keep track of TranslationUnit instance when annotating tokens
Previously the _tu was not propagated to the returned cursor, leading to errors when calling any
method on that cursor (e.g. cursor.referenced).
Reviewers: jbcoe, rsmith
Reviewed By: jbcoe
Patch by jklaehn (Johann Klähn)
Differential Revision: https://reviews.llvm.org/D36953