Comment parsing: use CharInfo.h

This also gives us 0.2% speedup on '-fsyntax-only -Wdocumentation' time for
a testcase that consists of all Clang headers.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174810 91177308-0d34-0410-b5e6-96231b3b80d8