clang-format: More restrictively classify import declarations.

Before:
  import::SomeFunction(aaaaaaaaaaaaaaaaaaaaaaaaaaa, aaaaaaaaaaaaaaaaaaaaaaaaaaaaa);

After:
  import::SomeFunction(aaaaaaaaaaaaaaaaaaaaaaaaaaa,
                       aaaaaaaaaaaaaaaaaaaaaaaaaaaaa);

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@223345 91177308-0d34-0410-b5e6-96231b3b80d8