Frontend: Parse diagnostic options earlier.

Diagnostics may be emitted while parsing command line arguments. This implies
that the options which affect how diagnostics are emitted and presented need to
be parsed first.

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -2248,10 +2248,6 @@ InterfaceSubContextDelegateImpl::runInSubCompilerInstance(StringRef moduleName,
     subInstance.addDiagnosticConsumer(&noopConsumer);
   }
 
-  // Eagerly suppress warnings if necessary, before parsing arguments.
-  if (subInvocation.getDiagnosticOptions().SuppressWarnings)
-    subInstance.getDiags().setSuppressWarnings(true);
-
   SwiftInterfaceInfo interfaceInfo;
   // Extract compiler arguments from the interface file and use them to configure
   // the compiler invocation.