Reland: [infra] Roll clang toolchain forward

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.
 
 declare_args() {
   # The optimization level to use for debug builds.
   if (is_android) {
     # On Android we kind of optimize some things that don't affect debugging
     # much even when optimization is disabled to get the binary size down.
     debug_optimization_level = "s"
@@ skipping 311 matching lines @@
       } else if (current_cpu == "x86") {
         cflags += [ "--target=i686-linux-androideabi" ]
         ldflags += [ "--target=i686-linux-androideabi" ]
       } else if (current_cpu == "x64") {
         cflags += [ "--target=x86_64-linux-androideabi" ]
         ldflags += [ "--target=x86_64-linux-androideabi" ]
       }
     }
   }
 
+  # We want to force a recompile and relink of the world whenever our toolchain
+  # changes since artifacts from an older version of the toolchain may or may
+  # not be compatible with newer ones. To achieve this, we insert a synthetic
+  # define into the compile line.
+  if (is_clang && (is_linux || is_mac)) {
+    if (is_linux) {
+      toolchain_stamp_file = "//buildtools/linux64/clang.stamp"
+    } else {
+      toolchain_stamp_file = "//buildtools/mac/clang.stamp"
+    }
+    toolchain_version = read_file(toolchain_stamp_file, "trim string")
+    defines = [ "TOOLCHAIN_VERSION=$toolchain_version" ]
+  }
+
   # Assign any flags set for the C compiler to asmflags so that they are sent
   # to the assembler. The Windows assembler takes different types of flags
   # so only do so for posix platforms.
   if (is_posix) {
     asmflags += cflags
     asmflags += cflags_c
   }
 }
 
 config("compiler_arm_fpu") {
@@ skipping 357 matching lines @@
       cflags = [ "/Zi" ]  # Produce PDB file, no edit and continue.
     }
     ldflags = [ "/DEBUG" ]
   } else {
     cflags = [
       "-g3",
       "-ggdb3",
     ]
   }
 }