clang: Instead of having update.sh clobber out/, add a define with the current revision.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 096f1a95a7fa287cf7ebc5642f6850b0edd59427..e5c984944c682157b6fca8ba4b60759c48aeb4f7 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -2316,6 +2316,14 @@
           ],
         },
       }],
+      ['clang==1 and OS!="win"', {
+        # This is here so that all files get recompiled after a clang roll and
+        # when turning clang on or off.
+        # (defines are passed via the command line, and build systems rebuild
+        # things when their commandline changes). Nothing should ever read this
+        # define.
+        'defines': ['CR_CLANG_REVISION=<!(<(DEPTH)/tools/clang/scripts/update.sh --print-revision)'],
+      }],
       ['enable_rlz==1', {
         'defines': ['ENABLE_RLZ'],
       }],