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.
486ec4bd74
