gn: include CR_CLANG_REVISION in defines for clang

build/config/compiler/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 if (cpu_arch == "arm") {
   import("//build/config/arm.gni")
 }
 if (is_posix) {
   import("//build/config/gcc/gcc_version.gni")
@@ skipping 85 matching lines @@
       cflags += [ "-fstack-protector", "--param=ssp-buffer-size=4" ]
     }
 
     # Linker warnings.
     if (!(is_chromeos && cpu_arch == "arm") && !is_mac) {
       # TODO(jochen): Enable this on chromeos on arm. http://crbug.com/356580
       ldflags += [ "-Wl,--fatal-warnings" ]
     }
   }
 
+  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) {
     # These flags are shared between the C compiler and linker.
     common_mac_flags = []
 
     # CPU architecture.
     if (cpu_arch == "x64") {
       common_mac_flags += [ "-arch", "x86_64" ]
     } else if (cpu_arch == "x86") {
@@ skipping 788 matching lines @@
       cflags += [ "-gsplit-dwarf" ]
     }
   }
 }
 
 config("no_symbols") {
   if (!is_win) {
     cflags = [ "-g0" ]
   }
 }