gn: define and use clang_base_path

build/toolchain/win/BUILD.gn

Index: build/toolchain/win/BUILD.gn
diff --git a/build/toolchain/win/BUILD.gn b/build/toolchain/win/BUILD.gn
index 097229debf87616cc22e016b2bcd112ec8e6d9ee..7d98b030d515368cf8f5238a9d60ef7c3b62fbc1 100644
--- a/build/toolchain/win/BUILD.gn
+++ b/build/toolchain/win/BUILD.gn
@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/clang/clang.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/win/visual_studio_version.gni")
 import("//build/toolchain/goma.gni")
@@ -70,8 +71,7 @@ template("msvc_toolchain") {
     } else {
       lld_link = "lld-link"
     }
-    prefix = rebase_path("//third_party/llvm-build/Release+Asserts/bin",
-                         root_build_dir)
+    prefix = rebase_path("$clang_base_path/bin", root_build_dir)
 
     # lld-link includes a replacement for lib.exe that can produce thin
     # archives and understands bitcode (for lto builds).
@@ -313,8 +313,7 @@ if (target_cpu == "x86") {
   msvc_toolchain("clang_x86") {
     environment = "environment.x86"
     toolchain_cpu = "x86"
-    prefix = rebase_path("//third_party/llvm-build/Release+Asserts/bin",
-                         root_build_dir)
+    prefix = rebase_path("$clang_base_path/bin", root_build_dir)
     cl = "${goma_prefix}$prefix/${clang_cl}"
     toolchain_os = "win"
     is_clang = true
@@ -358,8 +357,7 @@ template("win_x64_toolchains") {
   msvc_toolchain("clang_" + target_name) {
     environment = "environment.x64"
     toolchain_cpu = "x64"
-    prefix = rebase_path("//third_party/llvm-build/Release+Asserts/bin",
-                         root_build_dir)
+    prefix = rebase_path("$clang_base_path/bin", root_build_dir)
     cl = "${goma_prefix}$prefix/${clang_cl}"
     toolchain_os = "win"
     is_clang = true