Fixes https://github.com/llvm/llvm-project/issues/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
bc40b76b5b