[gn build] Allow use_asan=true on macOS

Seems to work.

(I only tried macOS, not iOS, but need to allow both because
the iOS toolchain used to build compiler-rt asserts otherwise.)

utils/gn/build/BUILD.gn

@@ -314,8 +314,9 @@ config("compiler_defaults") {
   }
 
   if (use_asan) {
-    assert(is_clang && current_os == "linux",
-           "asan only supported on Linux/Clang")
+    assert(is_clang && (current_os == "ios" || current_os == "linux" ||
+                            current_os == "mac"),
+           "asan only supported on iOS/Clang, Linux/Clang, and macOS/Clang")
     cflags += [ "-fsanitize=address" ]
     ldflags += [ "-fsanitize=address" ]
   }