IRGen: re-enable generate static arrays in read-only data sections.

So far, static arrays had to be put into a writable section, because the isa pointer and the (immortal) ref count field were initialized dynamically at the first use of such an array.

But with a new runtime library, which exports the symbols for the (immortal) ref count field and the isa pointer, it's possible to put the whole array into a read-only section. I.e. make it a constant global.

rdar://94185998

This reverts the revert commit df353ff3c0.
Also, I added a frontend option to disable this optimization: `-disable-readonly-static-objects`

lib/Frontend/CompilerInvocation.cpp

@@ -2329,6 +2329,10 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
     Opts.NoPreallocatedInstantiationCaches = true;
   }
 
+  if (Args.hasArg(OPT_disable_readonly_static_objects)) {
+    Opts.DisableReadonlyStaticObjects = true;
+  }
+
   // Default to disabling swift async extended frame info on anything but
   // darwin. Other platforms are unlikely to have support for extended frame
   // pointer information.