Disable Clang's warnings about undefined pointer comparisons in Debug builds

The address of a reference, or the 'this' pointer, are always non-NULL
in well-defined C/C++ programs, and Clang warns about comparing them to NULL,
because the comparisons may be optimized away.

However, it can still be useful to do such comparisons in Debug builds.
For example, Skia does a lot of SkASSERT(&a), where a is a reference
parameter.

BUG=404271

Review URL: https://codereview.chromium.org/479293002

git-svn-id: http://src.chromium.org/svn/trunk/src/build@290361 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -3231,6 +3231,30 @@
               ],
             },
           }],
+          ['clang==1', {
+            'cflags': [
+              # Allow comparing the address of references and 'this' against 0
+              # in debug builds. Technically, these can never be null in
+              # well-defined C/C++ and Clang can optimize such checks away in
+              # release builds, but they may be used in asserts in debug builds.
+              '-Wno-undefined-bool-conversion',
+              '-Wno-tautological-undefined-compare',
+            ],
+            'xcode_settings': {
+              'OTHER_CFLAGS': [
+                '-Wno-undefined-bool-conversion',
+                '-Wno-tautological-undefined-compare',
+              ],
+            },
+            'msvs_settings': {
+              'VCCLCompilerTool': {
+                'AdditionalOptions': [
+                  '-Wno-undefined-bool-conversion',
+                  '-Wno-tautological-undefined-compare',
+                ],
+              },
+            },
+          }],
         ],
       },
       'Release_Base': {