Make the DWARF version emitted by the Swift compiler configurable.

Previously it was hardcoded to version 4 on all platforms.
This patch introduces a driver and frontend option -dwarf-version to configure it if needed.

lib/FrontendTool/FrontendTool.cpp

@@ -2240,6 +2240,10 @@ int swift::performFrontend(ArrayRef<const char *> Args,
              trace.emplace(*buffer);
            });
 
+  // Setting DWARF Version based on frontend options.
+  IRGenOptions &IRGenOpts = Invocation.getIRGenOptions();
+  IRGenOpts.DWARFVersion = IRGenOpts.DWARFVersion;
+
   // The compiler invocation is now fully configured; notify our observer.
   if (observer) {
     observer->parsedArgs(Invocation);