[clang] [clang-format] Add MainIncludeChar option. (PR #78752)

[Björn Schäpers via cfe-commits]

================
@@ -151,6 +151,21 @@ struct IncludeStyle {
   /// before any other include.
   /// \version 10
   std::string IncludeIsMainSourceRegex;
+
+  /// Character to consider in the include directives for the main header.
+  enum MainIncludeCharDiscriminator {
----------------
HazardyKnusperkeks wrote:

```suggestion
  enum MainIncludeCharDiscriminator : int8_t {
```
Is now common in clang format.

https://github.com/llvm/llvm-project/pull/78752