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

[Owen Pan via cfe-commits]

================
@@ -27981,6 +27981,118 @@ TEST_F(FormatTest, BreakBinaryOperations) {
                Style);
 }
 
+TEST_F(FormatTest, ShortNamespacesOption) {
+  FormatStyle Style = getLLVMStyle();
+  Style.AllowShortNamespacesOnASingleLine = true;
+  Style.FixNamespaceComments = false;
+
+  // Basic functionality.
+  verifyFormat("namespace foo { class bar; }", Style);
+  verifyFormat("namespace foo::bar { class baz; }", Style);
+  verifyFormat("namespace { class bar; }", Style);
+  verifyFormat("namespace foo {\n"
+               "class bar;\n"
+               "class baz;\n"
+               "}",
+               Style);
+
+  // Trailing comments prevent merging.
+  verifyFormat("namespace foo {\n"
+               "namespace baz { class qux; } // comment\n"
+               "}",
+               Style);
+
+  // Make sure code doesn't walk to far on unbalanced code.
----------------
owenca wrote:

```suggestion
  // Make sure code doesn't walk too far on unbalanced code.
```

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