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

[Owen Pan via cfe-commits]

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@@ -1501,6 +1536,8 @@ FormatStyle getLLVMStyle(FormatStyle::LanguageKind Language) {
   LLVMStyle.AllowShortCompoundRequirementOnASingleLine = true;
   LLVMStyle.AllowShortEnumsOnASingleLine = true;
   LLVMStyle.AllowShortFunctionsOnASingleLine = FormatStyle::SFS_All;
+  LLVMStyle.AllowShortFunctionsOnASingleLineOptions = {};
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owenca wrote:

```suggestion
  LLVMStyle.AllowShortFunctionsOnASingleLineOptions.Empty = true;
  LLVMStyle.AllowShortFunctionsOnASingleLineOptions.Inline = true;
```
`SFS_All` requires setting everything.

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