Merge pull request #18616 from akyrtzi/input-complete-invalid-interpolation

[parser] Make corrections for the isInputIncomplete() functionality

lib/Parse/ParseExpr.cpp

@@ -1765,6 +1765,10 @@ ParserResult<Expr> Parser::parseExprPrimary(Diag<> ID, bool isExprBasic) {
   // Eat an invalid token in an expression context.  Error tokens are diagnosed
   // by the lexer, so there is no reason to emit another diagnostic.
   case tok::unknown:
+    if (Tok.getText().startswith("\"\"\"")) {
+      // This was due to unterminated multi-line string.
+      IsInputIncomplete = true;
+    }
     consumeToken(tok::unknown);
     return nullptr;