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

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<!--LLVM CODE FORMAT COMMENT: {clang-format}-->


:warning: C/C++ code formatter, clang-format found issues in your code. :warning:

<details>
<summary>
You can test this locally with the following command:
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``````````bash
git-clang-format --diff HEAD~1 HEAD --extensions cpp,h -- clang/include/clang/Format/Format.h clang/lib/Format/Format.cpp clang/lib/Format/TokenAnnotator.cpp clang/lib/Format/UnwrappedLineFormatter.cpp clang/unittests/Format/ConfigParseTest.cpp clang/unittests/Format/DefinitionBlockSeparatorTest.cpp clang/unittests/Format/FormatTest.cpp clang/unittests/Format/FormatTestCSharp.cpp clang/unittests/Format/FormatTestJS.cpp clang/unittests/Format/FormatTestJava.cpp
``````````

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<details>
<summary>
View the diff from clang-format here.
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``````````diff
diff --git a/clang/include/clang/Format/Format.h b/clang/include/clang/Format/Format.h
index e94531ea8..7994b51d5 100644
--- a/clang/include/clang/Format/Format.h
+++ b/clang/include/clang/Format/Format.h
@@ -836,7 +836,7 @@ struct FormatStyle {
   /// * ``InlineOnly``
   ///   Only merge functions defined inside a class. Same as ``inline``,
   ///   except it does not implies ``empty``: i.e. top level empty functions
-  ///   are not merged either. This option is **deprecated** and is retained 
+  ///   are not merged either. This option is **deprecated** and is retained
   ///   for backwards compatibility. See ``Inline`` of ``ShortFunctionStyle``.
   ///   \code
   ///     class Foo {
@@ -850,8 +850,8 @@ struct FormatStyle {
   ///   \endcode
   ///
   /// * ``Empty``
-  ///   Only merge empty functions. This option is **deprecated** and is 
-  ///   retained for backwards compatibility. See ``Empty`` of 
+  ///   Only merge empty functions. This option is **deprecated** and is
+  ///   retained for backwards compatibility. See ``Empty`` of
   ///   ``ShortFunctionStyle``.
   ///   \code
   ///     void f() {}

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