gn: define and use clang_base_path

build/config/clang/BUILD.gn

Index: build/config/clang/BUILD.gn
diff --git a/build/config/clang/BUILD.gn b/build/config/clang/BUILD.gn
index dbff398e1f6d06231396b432f7167d00bfc50e38..64e467acad9ad503af382178dafab1732d35d523 100644
--- a/build/config/clang/BUILD.gn
+++ b/build/config/clang/BUILD.gn
@@ -17,18 +17,16 @@ config("find_bad_constructs") {
         "-Xclang",
         "-load",
         "-Xclang",
-        rebase_path(
-            "//third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.dylib",
-            root_build_dir),
+        rebase_path("${clang_base_path}/lib/libFindBadConstructs.dylib",
+                    root_build_dir),
       ]
     } else if (is_linux || is_android) {
       cflags += [
         "-Xclang",
         "-load",
         "-Xclang",
-        rebase_path(
-            "//third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.so",
-            root_build_dir),
+        rebase_path("${clang_base_path}/lib/libFindBadConstructs.so",
+                    root_build_dir),
       ]
     }