Move operator precedence calculation to new header

Thereby, it can be reused by clang-format and others.
Review: http://llvm-reviews.chandlerc.com/D229

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170757 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed