Syntax: introduce a token kind for single quote to preserve round-trip printing of syntax tree

For invalid code, lexer is forgiving enough to allow single quote '\'' as the starting point
of string literals. Later, parser assumes the string literals are always using double quote "\"", and passes
such knowledge to SwiftSyntax side, leading to the round-trip failure we observed in the radar.
This commit fixes the issue by introducing another token kind for single quote.

rdar://51071021

Author: nkcsgexi

lib/Parse/ParseExpr.cpp

@@ -1963,8 +1963,9 @@ ParserResult<Expr> Parser::parseExprStringLiteral() {
   unsigned QuoteLength;
   tok QuoteKind;
   std::tie(QuoteLength, QuoteKind) =
-      Tok.isMultilineString() ? std::make_tuple(3, tok::multiline_string_quote)
-                              : std::make_tuple(1, tok::string_quote);
+    Tok.isMultilineString() ? std::make_tuple(3, tok::multiline_string_quote)
+                            : std::make_tuple(1, Tok.getText().startswith("\'") ?
+                                          tok::single_quote: tok::string_quote);
   unsigned CloseQuoteBegin = Tok.getLength() - DelimiterLength - QuoteLength;
 
   OpenDelimiterStr = Tok.getRawText().take_front(DelimiterLength);

test/Syntax/round_trip_fix_quote.swift

@@ -0,0 +1,7 @@
+// RUN: rm -rf %t
+// RUN: %swift-syntax-test -input-source-filename %s -parse-gen > %t
+// RUN: diff -u %s %t
+
+"abc"
+
+RoundedRectangle(cornerRadius: 6.0)lengthlength'Integer' field' [binding value: \(int.value)]int'Double' field [binding value: \(double.value)]double

utils/gyb_syntax_support/Token.py

@@ -249,6 +249,8 @@ def macro_name(self):
 
     Punctuator('StringQuote', 'string_quote', text='\\\"',
                classification='StringLiteral', serialization_code=102),
+    Punctuator('SingleQuote', 'single_quote', text='\\\'',
+               classification='StringLiteral', serialization_code=120),
     Punctuator('MultilineStringQuote', 'multiline_string_quote',
                text='\\\"\\\"\\\"', classification='StringLiteral',
                serialization_code=103),