[Syntax] Do not add a node for 'eof' into the tree

Summary:
While useful as a sentinel value when iterating over tokens, having
'eof' in the tree, seems to do more harm than good.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: javed.absar, kristof.beyls, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64576

llvm-svn: 368062