[Parser] fix parse trailing commna in string interpolation at the end of parseListItem flow instead

Author: kntkymt

lib/Parse/ParseExpr.cpp

@@ -1908,11 +1908,6 @@ ParserResult<Expr> Parser::parseExprPrimary(Diag<> ID, bool isExprBasic) {
   default:
   UnknownCharacter:
     checkForInputIncomplete();
-    // Enable trailing comma in string literal interpolation
-    // Note that 'Tok.is(tok::r_paren)' is not used because the text is ")\"\n" but the kind is actualy 'eof'
-    if (Tok.is(tok::eof) && Tok.getText() == ")") {
-      return nullptr;
-    }
     // FIXME: offer a fixit: 'Self' -> 'self'
     diagnose(Tok, ID);
     return nullptr;

lib/Parse/Parser.cpp

@@ -1057,11 +1057,18 @@ Parser::parseListItem(ParserStatus &Status, tok RightK, SourceLoc LeftLoc,
       return ParseListItemResult::Finished;
   }
   if (consumeIf(tok::comma)) {
-    if (Tok.isNot(RightK))
+    if (Tok.isNot(RightK) && !tokIsStringInterpolationEOF(Tok, RightK))
       return ParseListItemResult::Continue;
     if (!AllowSepAfterLast) {
       diagnose(Tok, diag::unexpected_separator, ",").fixItRemove(PreviousLoc);
     }
+    
+    // Enable trailing comma in string literal interpolation
+    if (tokIsStringInterpolationEOF(Tok, RightK)) {
+      RightLoc = Tok.getLoc();
+      return ParseListItemResult::FinishedInStringInterpolation;
+    }
+    
     return ParseListItemResult::Finished;
   }
   // If we're in a comma-separated list, the next token is at the