gn build: Silence mismatched-new-delete warning in scudo C++ wrapper tests.

These tests are deliberately mismatching new and delete, so the warnings
are just noise.

Differential Revision: https://reviews.llvm.org/D71783

utils/gn/secondary/compiler-rt/lib/scudo/standalone/tests/BUILD.gn

@@ -1,3 +1,4 @@
+import("//llvm/utils/gn/build/toolchain/compiler.gni")
 import("//llvm/utils/unittest/unittest.gni")
 
 unittest("ScudoUnitTest") {
@@ -54,5 +55,8 @@ unittest("ScudoCxxUnitTest") {
     "scudo_unit_test_main.cpp",
     "wrappers_cpp_test.cpp",
   ]
+  if (is_clang) {
+    cflags = [ "-Wno-mismatched-new-delete" ]
+  }
   has_custom_main = true
 }