[clang-format] Do not break after ObjC category open paren

Summary:
Previously, `clang-format` would break Objective-C
category extensions after the opening parenthesis to avoid
breaking the protocol list:

```
% echo "@interface ccccccccccccc (ccccccccccc) <ccccccccccccc> { }" | \
  clang-format -assume-filename=foo.h -style="{BasedOnStyle: llvm, \
  ColumnLimit: 40}"
@interface ccccccccccccc (
    ccccccccccc) <ccccccccccccc> {
}
```

This looks fairly odd, as we could have kept the category extension
on the previous line.

Category extensions are a single item, so they are generally very
short compared to protocol lists. We should prefer breaking after the
opening `<` of the protocol list over breaking after the opening `(`
of the category extension.

With this diff, we now avoid breaking after the category extension's
open paren, which causes us to break after the protocol list's
open angle bracket:

```
% echo "@interface ccccccccccccc (ccccccccccc) <ccccccccccccc> { }" | \
  ./bin/clang-format -assume-filename=foo.h -style="{BasedOnStyle: llvm, \
  ColumnLimit: 40}"
@interface ccccccccccccc (ccccccccccc) <
    ccccccccccccc> {
}
```

Test Plan: New test added. Confirmed test failed before diff and
  passed after diff by running:
  % make -j16 FormatTests && ./tools/clang/unittests/Format/FormatTests

Reviewers: djasper, jolesiak

Reviewed By: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D45526

llvm-svn: 329919