[clang] [clang-format]: Add `Custom` to `ShortFunctionStyle`; add new AllowShortFunctionsOnASingleLineOptions for granular setup (PR #134337)

[Owen Pan via cfe-commits]

================
@@ -871,13 +871,81 @@ struct FormatStyle {
     ///   void f() { bar(); }
     /// \endcode
     SFS_All,
+    /// Configure merge behavior using AllowShortFunctionsOnASingleLineOptions
+    SFS_Custom,
   };
 
   /// Dependent on the value, ``int f() { return 0; }`` can be put on a
   /// single line.
   /// \version 3.5
   ShortFunctionStyle AllowShortFunctionsOnASingleLine;
 
+  /// Precise control over merging short functions
+  /// \code
+  ///   # Should be declared this way:
+  ///   AllowShortFunctionsOnASingleLine: Custom
+  ///   AllowShortFunctionsOnASingleLineOptions:
+  ///     Empty: false
+  ///     Inline: true
+  ///     All: false
+  /// \endcode
+  struct ShortFunctionMergeFlags {
+    /// Only merge empty functions.
----------------
owenca wrote:

```suggestion
    /// Merge top-level empty functions.
```
Would it cover empty functions with comments? For example:
```
void g() { /**/
}
void h() {
  /**/
}
```

https://github.com/llvm/llvm-project/pull/134337