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

This fixes #101363 which is a resurrection of a previously opened but
never completed review: https://reviews.llvm.org/D11851

The feature is to allow code like the following not to be broken across
multiple lines:

```
namespace foo { class bar; }
namespace foo { namespace bar { class baz; } }
```

Code like this is commonly used for forward declarations, which are
ideally kept compact. This is also apparently the format that
include-what-you-use will insert for forward declarations.

Also, fix an off-by-one error in `CompactNamespaces` code. For nested
namespaces with 3 or more namespaces, it was incorrectly compacting
lines which were 1 or two spaces over the `ColumnLimit`, leading to
incorrect formatting results.

1 files changed, 1 insertions, 0 deletions

diff --git a/clang/unittests/Format/ConfigParseTest.cpp b/clang/unittests/Format/ConfigParseTest.cpp
index 7fc7492..b249bf0 100644
--- a/clang/unittests/Format/ConfigParseTest.cpp
+++ b/clang/unittests/Format/ConfigParseTest.cpp
@@ -159,6 +159,7 @@ TEST(ConfigParseTest, ParsesConfigurationBools) {
   CHECK_PARSE_BOOL(AllowShortCompoundRequirementOnASingleLine);
   CHECK_PARSE_BOOL(AllowShortEnumsOnASingleLine);
   CHECK_PARSE_BOOL(AllowShortLoopsOnASingleLine);
+  CHECK_PARSE_BOOL(AllowShortNamespacesOnASingleLine);
   CHECK_PARSE_BOOL(BinPackArguments);
   CHECK_PARSE_BOOL(BreakAdjacentStringLiterals);
   CHECK_PARSE_BOOL(BreakAfterJavaFieldAnnotations);