[clang-repl] Use RegisterPTU for tracking generated TranslationUnitDecl through parse (llvm#164778)

Instead of manually creating and adding a PTU, we should be able to use
`RegisterPTU` which does the same job here.

Author: anutosh491

clang/lib/Interpreter/Interpreter.cpp

@@ -581,12 +581,7 @@ Interpreter::Parse(llvm::StringRef Code) {
   if (!TuOrErr)
     return TuOrErr.takeError();
 
-  PTUs.emplace_back(PartialTranslationUnit());
-  PartialTranslationUnit &LastPTU = PTUs.back();
-  LastPTU.TUPart = *TuOrErr;
-
-  if (std::unique_ptr<llvm::Module> M = Act->GenModule())
-    LastPTU.TheModule = std::move(M);
+  PartialTranslationUnit &LastPTU = IncrParser->RegisterPTU(*TuOrErr);
 
   return LastPTU;
 }