gn: include CR_CLANG_REVISION in defines for clang

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 793ee6a033a653c8e9553fdfe53da0a8a8cd75eb..a028841c24556fa58743f9604f2303f5fcfd1d8f 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -103,6 +103,18 @@ config("compiler") {
     }
   }
 
+  if (is_clang && !is_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 system rebuild things when their commandline changes). Nothing
+    # should ever read this define.
+    defines += [
+      "CR_CLANG_REVISION=" +
+          exec_script(
+              "//tools/clang/scripts/posix-print-revision.py", [], "value")
+    ]
+  }
+
   # Mac-specific compiler flags setup.
   # ----------------------------------
   if (is_mac || is_ios) {