[clang-repl] Support compound statement as a top-level statement.

[v.g.vassilev]

Summary

This patch teaches our incremental compilation infrastructure to push and pop a fake function scope making the Parser happy when parsing compound statements as part of a top-leve statement declaration.

[v.g.vassilev]

ping

[v.g.vassilev]

This seems like a good candidate for a post commit review.

clang/lib/Parse/ParseDecl.cpp

 }
 
 Parser::DeclGroupPtrTy Parser::ParseTopLevelStmtDecl() {
   assert(PP.isIncrementalProcessingEnabled() && "Not in incremental mode");
 
   // Parse a top-level-stmt.
   Parser::StmtVector Stmts;
   ParsedStmtContext SubStmtCtx = ParsedStmtContext();
+  Actions.PushFunctionScope();
   StmtResult R = ParseStatementOrDeclaration(Stmts, SubStmtCtx);
+  Actions.PopFunctionScopeInfo();
   if (!R.isUsable())
     return nullptr;
 
   SmallVector<Decl *, 2> DeclsInGroup;
   DeclsInGroup.push_back(Actions.ActOnTopLevelStmtDecl(R.get()));
   // Currently happens for things like  -fms-extensions and use `__if_exists`.
   for (Stmt *S : Stmts)
     DeclsInGroup.push_back(Actions.ActOnTopLevelStmtDecl(S));

clang/test/Interpreter/execute-stmts.cpp

 ::g();
 
 printf("i = %d\n", i);
 // CHECK-NEXT: i = 5
 
 for (; i > 4; --i) printf("i = %d\n", i);
 // CHECK-NEXT: i = 5
 
+{++i;}
+
+for (; i > 4; --i) { printf("i = %d\n", i); };
+// CHECK-NEXT: i = 5
+
 int j = i; printf("j = %d\n", j);
 // CHECK-NEXT: j = 4