Introducing -Wheader-guard, a warning that checks header guards actually work
properly.  This warning checks that the #ifndef and #define directives at
the beginning of a header refer to the same macro name.  Includes a fix-it
hint to correct the header guard.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183867 91177308-0d34-0410-b5e6-96231b3b80d8