gn build: Add a `use_asan` build arg that can be used to build asanified clangs and llds

[thakis]

Summary

As-is, this also builds an asanified hwasan runtime. Not sure if that's good or not.

[pcc]

We shouldn't be compiling any of the sanitizer runtimes with sanitizers, that's pretty much guaranteed not to work.

We could do something like this to disable asan in non-default toolchains:
https://cs.chromium.org/chromium/src/build/config/sanitizers/sanitizers.gni?l=123

llvm/utils/gn/build/BUILD.gn

 import("//llvm/utils/gn/build/buildflags.gni")
 import("//llvm/utils/gn/build/mac_sdk.gni")
 import("//llvm/utils/gn/build/toolchain/compiler.gni")
 import("//llvm/utils/gn/build/toolchain/target_flags.gni")
 
+declare_args() {
+  # Build all targets with -fsanitize=address.
+  use_asan = false
+}
+
 config("compiler_defaults") {
   defines = []
 
   if (!llvm_enable_assertions) {
     defines += [ "NDEBUG" ]
   }
 
   asmflags = target_flags
if (host_os != "win") {
     if (is_optimized) {
       cflags += [ "-O3" ]
     }
     cflags += [ "-fdiagnostics-color" ]
     cflags_cc = [
       "-std=c++11",
       "-fvisibility-inlines-hidden",
     ]
+
+    if (use_asan) {
+      cflags += [ "-fsanitize=address" ]
+      ldflags += [ "-fsanitize=address" ]
+    }
   } else {
+    assert(!use_asan, "use_asan=true not yet implemented for Windows")
     if (is_debug) {
       cflags += [
         "/Zi",
         "/FS",
       ]
       ldflags += [ "/DEBUG" ]
     }
     if (is_optimized) {