clang tot bots: let mac/win asan bots use non-stale asan runtimes

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index 48e3671a047828687bbf287b427578d9d7907f38..5722c125d0073196a0476e37541ecf9f9493aa9e 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -50,7 +50,12 @@ hermetic_xcode_path =
 declare_args() {
   if (is_clang) {
     # Clang compiler version. Clang files are placed at version-dependent paths.
-    clang_version = "4.0.0"
+    if (llvm_force_head_revision) {
+      clang_version = "5.0.0"
+    } else {
+      # TODO(thakis): Remove this branch the next time clang rolls.
+      clang_version = "4.0.0"
+    }
   }
 }