[clang-format] Stop crashing when the input contains ??/\n

In the debug build, this kind of input caused the assertion in the
function `countLeadingWhitespace` to fail.  The release build without
assertions outputted `?` `?` `/` separated by spaces.

```C
#define A ??/
  int i;
```

This is because the code in `countLeadingWhitespace` assumed that the
underlying lexer recognized the entire `??/` sequence as a single token.
In fact, the lexer recognized it as 3 separate tokens.  The flag to make
the lexer recognize trigraphs was never enabled.

This patch enables the flag in the underlying lexer.  This way, the
program now either turns the trigraph into a single `\` or removes it
altogether if the line is short enough.  There are operators like the
`??=` in C#.  So the flag is not enabled for all input languages.
Instead the check for the token size is moved from the assert line into
the if line.

The problem was introduced by my own patch 370bee4 from about 3
years ago.  I added code to count the number of characters in the escape
sequence probably just because the block of code used to have a comment
saying someone should add the feature.  Maybe I forgot to enable
assertions when I ran the code.  I found the problem because reviewing
pull request 145243 made me look at the code again.

Author: sstwcw

clang/lib/Format/Format.cpp

@@ -4052,6 +4052,7 @@ LangOptions getFormattingLangOpts(const FormatStyle &Style) {
   // the sequence "<::" will be unconditionally treated as "[:".
   // Cf. Lexer::LexTokenInternal.
   LangOpts.Digraphs = SinceCpp11;
+  LangOpts.Trigraphs = Style.isCpp();
 
   LangOpts.LineComment = 1;
   LangOpts.Bool = 1;

clang/lib/Format/FormatTokenLexer.cpp

@@ -1220,12 +1220,11 @@ static size_t countLeadingWhitespace(StringRef Text) {
         break;
       // Splice found, consume it.
       Cur = Lookahead + 1;
-    } else if (Cur[0] == '?' && Cur[1] == '?' && Cur[2] == '/' &&
-               (Cur[3] == '\n' || Cur[3] == '\r')) {
+    } else if (End - Cur >= 4u && Cur[0] == '?' && Cur[1] == '?' &&
+               Cur[2] == '/' && (Cur[3] == '\n' || Cur[3] == '\r')) {
       // Newlines can also be escaped by a '?' '?' '/' trigraph. By the way, the
       // characters are quoted individually in this comment because if we write
       // them together some compilers warn that we have a trigraph in the code.
-      assert(End - Cur >= 4);
       Cur += 4;
     } else {
       break;
@@ -1300,8 +1299,11 @@ FormatToken *FormatTokenLexer::getNextToken() {
       case '\\':
       case '?':
       case '/':
-        // The text was entirely whitespace when this loop was entered. Thus
-        // this has to be an escape sequence.
+        // The code preceding the loop and in the countLeadingWhitespace
+        // function guarantees that Text is entirely whitespace, not including
+        // comments but including escaped newlines which may be escaped with a
+        // trigraph. So if 1 of these characters show up, then it has to be in
+        // an escape sequence.
         assert(Text.substr(i, 4) == "\?\?/\r" ||
                Text.substr(i, 4) == "\?\?/\n" ||
                (i >= 1 && (Text.substr(i - 1, 4) == "\?\?/\r" ||

clang/unittests/Format/FormatTest.cpp

@@ -6681,6 +6681,31 @@ TEST_F(FormatTest, EscapedNewlines) {
                "  int x(int a);",
                AlignLeft);
 
+  // Escaped with a trigraph.
+  verifyFormat("#define A \\\n"
+               "  int i;  \\\n"
+               "  int j;",
+               "#define A \?\?/\n"
+               "int i;\?\?/\n"
+               "  int j;",
+               Narrow);
+  verifyFormat("#define A \\\r\n"
+               "  int i;  \\\r\n"
+               "  int j;",
+               "#define A \?\?/\r\n"
+               "int i;\?\?/\r\n"
+               "  int j;",
+               Narrow);
+  verifyFormat("#define A int i;", "#define A \?\?/\n"
+                                   "int i;");
+  verifyFormat("#define A int i;", "#define A \?\?/\r\n"
+                                   "int i;");
+  // In a language that does not support the trigraph, the program should not
+  // crash.
+  verifyNoCrash("#define A \?\?/\n"
+                "int i;",
+                getGoogleStyle(FormatStyle::LK_CSharp));
+
   // CRLF line endings
   verifyFormat("#define A \\\r\n  int i;  \\\r\n  int j;",
                "#define A \\\r\nint i;\\\r\n  int j;", Narrow);