Silence some MSVC warnings.

[sas]

Summary

Just assert("string" && false) instead of assert("string" == NULL).

This avoid errors like

[...]\Core\SourceManager.cpp(647): warning C4130: '==' : logical operation on address of string constant

[zturner]

Thanks for fixing these. Feel free to commit trivial warning fixes without
review.

source/Core/SourceManager.cpp

if (line == UINT32_MAX)
                         m_offsets.push_back(end - start);
                 }
                 return true;
             }
         }
         else
         {
             // Some lines have been populated, start where we last left off
-            assert("Not implemented yet" == NULL);
+            assert("Not implemented yet" && false);
         }
 
     }
     else
     {
         // Calculate all line offsets up to "line"
-        assert("Not implemented yet" == NULL);
+        assert("Not implemented yet" && false);
     }
     return false;
 }
 
 bool
 SourceManager::File::GetLine (uint32_t line_no, std::string &buffer)
 {
     if (!LineIsValid(line_no))