Reland: [infra] Roll clang toolchain forward

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 52256f5dfc047fb9b499b7b21741ddd01a9a1538..b999f2b52b100e6fc67940dabcfda92d03be7e4e 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -329,6 +329,20 @@ config("compiler") {
     }
   }
 
+  # 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.