Flip the order the preprocessor and frontendaction are informed of the end of a file.

This allows using EndOfMainFile from a PPCallback to access data from the
action. The pattern of PPCallback referencing an action is common in clang-tidy.

Differential Revision: http://reviews.llvm.org/D4773

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215145 91177308-0d34-0410-b5e6-96231b3b80d8