Merge NullDerefChecker.[h,cpp] and UndefDerefChecker.[h,cpp]. They are...

Merge NullDerefChecker.[h,cpp] and UndefDerefChecker.[h,cpp].  They are essentially two parts of the same check.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@85911 91177308-0d34-0410-b5e6-96231b3b80d8