gn build: check-clang depends on llvm-cxxfilt.

Author: pcc

llvm/utils/gn/secondary/clang/test/BUILD.gn

@@ -142,6 +142,7 @@ group("test") {
     "//llvm/tools/llvm-bcanalyzer",
     "//llvm/tools/llvm-cat",
     "//llvm/tools/llvm-config",
+    "//llvm/tools/llvm-cxxfilt",
     "//llvm/tools/llvm-dis",
     "//llvm/tools/llvm-dwarfdump",
     "//llvm/tools/llvm-ifs",