gn build: Split no-RTTI flag into a config.

[pcc]

Summary

Some of the sanitizer runtime code needs to be built with RTTI;
this allows that code to opt in to RTTI.

llvm/utils/gn/build/BUILD.gn

if (is_debug) {
       cflags += [ "-g" ]
     } else {
       cflags += [ "-O3" ]
     }
     cflags += [ "-fdiagnostics-color" ]
     cflags_cc = [
       "-std=c++11",
       "-fno-exceptions",
-      "-fno-rtti",
       "-fvisibility-inlines-hidden",
     ]
   } else {
     if (is_debug) {
       cflags += [ "/Zi" ]
     } else {
       cflags += [
         "/O2",
         "/Zc:inline",
       ]
     }
     defines += [
       "_CRT_SECURE_NO_DEPRECATE",
       "_CRT_SECURE_NO_WARNINGS",
       "_CRT_NONSTDC_NO_DEPRECATE",
       "_CRT_NONSTDC_NO_WARNINGS",
       "_SCL_SECURE_NO_DEPRECATE",
       "_SCL_SECURE_NO_WARNINGS",
 
       "_HAS_EXCEPTIONS=0",
       "_UNICODE",
       "UNICODE",
     ]
-    cflags += [
-      "/EHs-c-",
-      "/GR-",
-    ]
+    cflags += [ "/EHs-c-" ]
 
     # The MSVC default value (1 MB) is not enough for parsing recursive C++
     # templates in Clang.
     ldflags = [ "/STACK:10000000" ]
   }
 
   # Warning setup.
   if (host_os == "win" && !is_clang) {
if (host_os == "win" && !is_clang) {
     if (is_clang && use_goma) {
       # goma converts all paths to lowercase on the server, breaking this
       # warning.
       cflags += [ "-Wno-nonportable-include-path" ]
     }
   }
 }
 
+config("no_rtti") {
+  if (current_os == "win") {
+    cflags_cc = [ "/GR-" ]
+  } else {
+    cflags_cc = [ "-fno-rtti" ]
+  }
+}
+
 config("llvm_code") {
   include_dirs = [
     "//llvm/include",
     "$root_gen_dir/llvm/include",
   ]
 }
 
 config("lld_code") {

llvm/utils/gn/build/BUILDCONFIG.gn

 # All targets will get this list of configs by default.
 # Targets can opt out of a config by removing it from their local configs list.
 # If you're adding global flags and don't need targets to be able to opt out,
 # add the flags to compiler_defaults, not to a new config.
 shared_binary_target_configs = [
   "//llvm/utils/gn/build:compiler_defaults",
   "//llvm/utils/gn/build:llvm_code",
+  "//llvm/utils/gn/build:no_rtti",
   "//llvm/utils/gn/build:warn_covered_switch_default",
 ]
 
 # Apply that default list to the binary target types.
 set_defaults("executable") {
   configs = shared_binary_target_configs
 }
 set_defaults("loadable_module") {