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

[Owen Pan via cfe-commits]

================
@@ -293,15 +293,14 @@ class LineJoiner {
 
     auto ShouldMergeShortFunctions = [this, &I, &NextLine, PreviousLine,
                                       TheLine]() {
-      if (Style.AllowShortFunctionsOnASingleLine == FormatStyle::SFS_All)
+      if (Style.AllowShortFunctionsOnASingleLine.Other)
         return true;
-      if (Style.AllowShortFunctionsOnASingleLine >= FormatStyle::SFS_Empty &&
+      if (Style.AllowShortFunctionsOnASingleLine.Empty &&
           NextLine.First->is(tok::r_brace)) {
         return true;
       }
 
-      if (Style.AllowShortFunctionsOnASingleLine &
-          FormatStyle::SFS_InlineOnly) {
+      if (Style.AllowShortFunctionsOnASingleLine.Inline) {
----------------
owenca wrote:

The new `Other` (e.g. top-level non-empty function) is not the same as the old `SFS_All`, so we still need to check `!Other` here, I think.

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