Relax header guard mismatch warning with edit distance heuristic.

If the edit distance between the two macros is more than 50%, DefinedMacro may not be header guard or can be header guard of another header file or it might be defining something completely different set by the build environment.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@192547 91177308-0d34-0410-b5e6-96231b3b80d8