[clang] [clang-format] Add `AllowShortNamespacesOnASingleLine` option (PR #105597)

[Owen Pan via cfe-commits]

================
@@ -28314,6 +28320,116 @@ TEST_F(FormatTest, KeepFormFeed) {
                Style);
 }
 
+TEST_F(FormatTest, ShortNamespacesOption) {
+  auto BaseStyle = getLLVMStyle();
+  BaseStyle.AllowShortNamespacesOnASingleLine = true;
+  BaseStyle.FixNamespaceComments = false;
+  BaseStyle.CompactNamespaces = true;
+
+  auto Style = BaseStyle;
----------------
owenca wrote:

```suggestion
  auto Style = getLLVMStyle();
  Style.AllowShortNamespacesOnASingleLine = true;
  Style.CompactNamespaces = true;
  Style.FixNamespaceComments = false;
```

https://github.com/llvm/llvm-project/pull/105597