Stop parsing into IfConfiDecl nodes in the C++ parser

When parsing #if...#endif regions, parse the active clause directly into
place in the AST without ever producing an IfConfigDecl instance.

lib/Parse/Parser.cpp

@@ -822,9 +822,11 @@ bool Parser::parseEndIfDirective(SourceLoc &Loc) {
     Loc = PreviousLoc;
     skipUntilConditionalBlockClose();
     return true;
-  } else if (!Tok.isAtStartOfLine() && Tok.isNot(tok::eof))
+  } else if (!Tok.isAtStartOfLine() && Tok.isNot(tok::eof)) {
     diagnose(Tok.getLoc(),
              diag::extra_tokens_conditional_compilation_directive);
+    skipUntilTokenOrEndOfLine(tok::NUM_TOKENS);
+  }
   return false;
 }