[clang-format] Fix BraceWrapping AfterFunction for ObjC methods

Summary:
> clang-format --version
> clang-format version 7.0.0 (tags/RELEASE_700/final)
> echo "@implementation Foo\n- (void)foo:(id)bar\n{\n}\n@end\n" |clang-format -style='{BreakBeforeBraces: Custom, BraceWrapping: {AfterFunction: true}}'
```
@implementation Foo
- (void)foo:(id)bar {
}
@end
```

with patch:

> bin/clang-format --version
> clang-format version 8.0.0 (trunk 344285)
> echo "@implementation Foo\n- (void)foo:(id)bar\n{\n}\n@end\n" |bin/clang-format -style='{BreakBeforeBraces: Custom, BraceWrapping: {AfterFunction: true}}'
```
@implementation Foo
- (void)foo:(id)bar
{
}
@end
```

Contributed by hultman@.

Reviewers: benhamilton, jolesiak, klimek, Wizard

Reviewed By: benhamilton

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D53197

llvm-svn: 344406