gn: Update clang_version when using ToT Clang

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index f0e455282c1959d85da4564101719433f52cbad5..f389eec29e41bf0d97c8a8ade6b867d497fe386b 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -22,10 +22,13 @@ declare_args() {
   # build/config/compiler/BUILD.gn).
   use_lld = is_win && host_os != "win"
 
-  if (is_clang) {
-    # Clang compiler version. Clang files are placed at version-dependent paths.
-    clang_version = "3.9.0"
-  }
+  # If this is set to true, or if LLVM_FORCE_HEAD_REVISION is set to 1
+  # in the environment, we use the revision in the llvm repo to determine
+  # the CLANG_REVISION to use, instead of the version hard-coded into
+  # //tools/clang/scripts/update.py. This should only be used in
+  # conjunction with setting LLVM_FORCE_HEAD_REVISION in the
+  # environment when `gclient runhooks` is run as well.
+  llvm_force_head_revision = false
 
   # Compile with Xcode version of clang instead of hermetic version shipped
   # with the build. Used on iOS to ship official builds (as they are built
@@ -33,6 +36,17 @@ declare_args() {
   use_xcode_clang = is_ios && is_official_build
 }
 
+if (is_clang) {
+  # Clang compiler version. Clang files are placed at version-dependent paths.
+  clang_version = "3.9.0"
+
+  if (llvm_force_head_revision) {
+    # TODO(hans): Remove after next Clang roll.
+    # ToT Clang has a higher version number.
+    clang_version = "4.0.0"
+  }
+}
+
 assert(!use_xcode_clang || is_ios,
        "Using Xcode's clang is only supported in iOS builds")