[Frontend] Remove performParseOnly

Most clients were only using it to populate the
main module with files, which is now done by
`getMainModule`. Instead, they can now just rely
on parsing happening lazily.

tools/swift-syntax-test/swift-syntax-test.cpp

@@ -636,9 +636,6 @@ int parseFile(
   assert(BufferIDs.size() == 1 && "Only expecting to process one source file");
   unsigned BufferID = BufferIDs.front();
 
-  // Parse the actual source file
-  Instance.performParseOnly();
-
   SourceFile *SF = nullptr;
   for (auto Unit : Instance.getMainModule()->getFiles()) {
     SF = dyn_cast<SourceFile>(Unit);
@@ -648,6 +645,9 @@ int parseFile(
   }
   assert(SF && "No source file");
 
+  // Force parsing to populate the syntax cache.
+  (void)SF->getSyntaxRoot();
+
   // In case the action specific callback succeeds, we output this error code
   int InternalExitCode = EXIT_SUCCESS;