[clang] [Clang][Preprocessor] Unify header-name lookahead for import and include (PR #191004)

[Hubert Tong via cfe-commits]

================
@@ -422,6 +422,7 @@ Bug Fixes in This Version
 - Fixed a crash on _BitInt(N) arrays where 129 ≤ N ≤ 192 due to incorrect array filler lowering. (#GH189643)
 - Fixed the behavior in C23 of ``auto``, by emitting an error when an array type is specified for a ``char *``. (#GH162694)
 - Fixed incorrect rejection of ``auto`` with reordered declaration specifiers in C23. (#GH164121)
+- Fixed incorrect handling of header-name lookahead in C++ import and #include directives involving digraphs and macro expansion. (#GH190693)
----------------
hubert-reinterpretcast wrote:

Perhaps the source compatibility implications for
```c
#define CONFIG_HEADER <:config.h>
#include CONFIG_HEADER
```
could be covered in more detail.

>From a brief search, the most likely source of such things is apparently tied to classic Mac OS.

https://github.com/llvm/llvm-project/pull/191004