[Immediate] Lazy Parsing

Implement lazy parsing, allowing functions to be
parsed only when they are executed.

Author: Zak Kent

lib/Frontend/Frontend.cpp

@@ -1576,6 +1576,10 @@ CompilerInstance::getSourceFileParsingOptions(bool forPrimary) const {
       frontendOpts.ReuseFrontendForMultipleCompilations) {
     opts |= ParsingFlags::EnableInterfaceHash;
   }
+  if (action == ActionType::Immediate) {
+    opts -= ParsingFlags::DisableDelayedBodies;
+    opts -= ParsingFlags::ValidateNewParserDiagnostics;
+  }
   return opts;
 }