Don't put short namespace on a single line.

Before:
namespace abc { class SomeClass; }
namespace def { void someFunction() {} }

After:
namespace abc {
class Def;
}
namespace def {
void someFunction() {}
}

Rationale:
a) Having anything other than forward declaration on the same line
   as a namespace looks confusing.
b) Formatting namespace-forward-declaration-combinations different
   from other stuff is inconsistent.
c) Wasting vertical space close to such forward declarations really
   does not affect readability.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181887 91177308-0d34-0410-b5e6-96231b3b80d8