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

[Owen Pan via cfe-commits]

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@@ -1500,7 +1523,10 @@ FormatStyle getLLVMStyle(FormatStyle::LanguageKind Language) {
   LLVMStyle.AllowShortCaseLabelsOnASingleLine = false;
   LLVMStyle.AllowShortCompoundRequirementOnASingleLine = true;
   LLVMStyle.AllowShortEnumsOnASingleLine = true;
-  LLVMStyle.AllowShortFunctionsOnASingleLine = FormatStyle::SFS_All;
+  LLVMStyle.AllowShortFunctionsOnASingleLine =
+      FormatStyle::ShortFunctionStyle({/*Empty=*/true,
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owenca wrote:

You mean `setEmptyOnly()` or similar? That would be a good idea.

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