clang/win: For 64-bit builds, enable warnings-as-errors and stop falling back to CL.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index c242572c8c125f366fbed752f65d007f94c8f7ee..21cd9c6d0cf2b2c961566f9a15f07a8f6cebf8bd 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -5476,15 +5476,12 @@
             }],
           ],
           'conditions': [
+            # Building with Clang on Windows is a work in progress and very
+            # experimental. See crbug.com/82385.
             ['clang==1', {
-              # Building with Clang on Windows is a work in progress and very
-              # experimental. See crbug.com/82385.
               'VCCLCompilerTool': {
-                'WarnAsError': 'false',
-                'RuntimeTypeInfo': 'false',
                 'AdditionalOptions': [
                   '-fmsc-version=1800',
-                  '/fallback',
 
                   # Many files use intrinsics without including this header.
                   # TODO(hans): Fix those files, or move this to sub-GYPs.
@@ -5527,6 +5524,14 @@
                 ],
               },
             }],
+            ['clang==1 and target_arch=="ia32"', {
+              'VCCLCompilerTool': {
+                'WarnAsError': 'false',
+                'AdditionalOptions': [
+                  '/fallback',
+                ],
+              },
+            }],
           ],
         },
       },