[clang-format] Correctly parse C99 digraphs: "<:", ":>", "<%", "%>", "%:", "%:%:".

Fixes #31592.

This commits enables lexing of digraphs in C++11 and onwards.
Enabling them in C++03 is error-prone, as it would unconditionally treat sequences like "<:" as digraphs, even if they are followed by a single colon, e.g. "<::" would be treated as "[:" instead of "<" followed by "::". Lexing in C++11 doesn't have this problem as it looks ahead the following token.
The relevant excerpt from Lexer::LexTokenInternal:
```
        // C++0x [lex.pptoken]p3:
        //  Otherwise, if the next three characters are <:: and the subsequent
        //  character is neither : nor >, the < is treated as a preprocessor
        //  token by itself and not as the first character of the alternative
        //  token <:.
```

Also, note that both clang and gcc turn on digraphs by default (-fdigraphs), so clang-format should match this behaviour.

Reviewed By: MyDeveloperDay, HazardyKnusperkeks, owenpan

Differential Revision: https://reviews.llvm.org/D118706

Author: mkurdej

clang/lib/Format/Format.cpp

@@ -3242,6 +3242,10 @@ LangOptions getFormattingLangOpts(const FormatStyle &Style) {
   LangOpts.CPlusPlus17 = LexingStd >= FormatStyle::LS_Cpp17;
   LangOpts.CPlusPlus20 = LexingStd >= FormatStyle::LS_Cpp20;
   LangOpts.Char8 = LexingStd >= FormatStyle::LS_Cpp20;
+  // Turning on digraphs in standards before C++0x is error-prone, because e.g.
+  // the sequence "<::" will be unconditionally treated as "[:".
+  // Cf. Lexer::LexTokenInternal.
+  LangOpts.Digraphs = LexingStd >= FormatStyle::LS_Cpp11;
 
   LangOpts.LineComment = 1;
   bool AlternativeOperators = Style.isCpp();

clang/unittests/Format/FormatTest.cpp

@@ -24219,6 +24219,16 @@ TEST_F(FormatTest, AlignAfterOpenBracketBlockIndentForStatement) {
                Style);
 }
 
+TEST_F(FormatTest, UnderstandsDigraphs) {
+  verifyFormat("int arr<:5:> = {};");
+  verifyFormat("int arr[5] = <%%>;");
+  verifyFormat("int arr<:::qualified_variable:> = {};");
+  verifyFormat("int arr[::qualified_variable] = <%%>;");
+  verifyFormat("%:include <header>");
+  verifyFormat("%:define A x##y");
+  verifyFormat("#define A x%:%:y");
+}
+
 } // namespace
 } // namespace format
 } // namespace clang