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/IRGen/IRGenModule.cpp

@@ -1785,6 +1785,19 @@ bool IRGenModule::shouldPrespecializeGenericMetadata() {
          canPrespecializeTarget;
 }
 
+bool IRGenModule::canMakeStaticObjectsReadOnly() {
+  if (getOptions().DisableReadonlyStaticObjects)
+    return false;
+
+  // TODO: Support constant static arrays on other platforms, too.
+  // See also the comment in GlobalObjects.cpp.
+  if (!Triple.isOSDarwin())
+    return false;
+
+  return getAvailabilityContext().isContainedIn(
+          Context.getImmortalRefCountSymbolsAvailability());
+}
+
 void IRGenerator::addGenModule(SourceFile *SF, IRGenModule *IGM) {
   assert(GenModules.count(SF) == 0);
   GenModules[SF] = IGM;