[clang-format] Use number of unwrapped lines for short namespace

Summary:
This patch makes the namespace comment fixer use the number of unwrapped lines
that a namespace spans to detect it that namespace is short, thus not needing
end comments to be added.
This is needed to ensure clang-format is idempotent. Previously, a short namespace
was detected by the original source code lines. This has the effect of requiring two
runs for this example:
```
namespace { class A; }
```
after first run:
```
namespace {
class A;
}
```
after second run:
```
namespace {
class A;
} // namespace
```

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D30528

llvm-svn: 296736