[clang] [BitInt] Expose a _BitInt literal suffix in C++ (PR #86586)

[Aaron Ballman via cfe-commits]

================
@@ -1241,7 +1260,9 @@ bool NumericLiteralParser::isValidUDSuffix(const LangOptions &LangOpts,
     return false;
 
   // By C++11 [lex.ext]p10, ud-suffixes starting with an '_' are always valid.
-  if (Suffix[0] == '_')
+  // Suffixes starting with '__' (double underscore) are for use by
+  // implementation.
+  if (Suffix[0] == '_' && Suffix[1] != '_')
----------------
AaronBallman wrote:

Example code that causes an assertion now:
```
unsigned operator ""_(const char *);

unsigned i = 1_;
```


https://github.com/llvm/llvm-project/pull/86586