[clang-format/ObjC] Correctly parse Objective-C methods with 'class' in name

Summary:
Please take a close look at this CL. I haven't touched much of
`UnwrappedLineParser` before, so I may have gotten things wrong.

Previously, clang-format would incorrectly format the following:

```
@implementation Foo

- (Class)class {
}

- (void)foo {
}

@end
```

as:

```
@implementation Foo

- (Class)class {
}

    - (void)foo {
}

@end
```

The problem is whenever `UnwrappedLineParser::parseStructuralElement()`
sees any of the keywords `class`, `struct`, or `enum`, it calls
`parseRecord()` to parse them as a C/C++ record.

This causes subsequent lines to be parsed incorrectly, which
causes them to be indented incorrectly.

In Objective-C/Objective-C++, these keywords are valid selector
components.

This diff fixes the issue by explicitly handling `+` and `-` lines
inside `@implementation` / `@interface` / `@protocol` blocks
and parsing them as Objective-C methods.

Test Plan: New tests added. Ran tests with:
  make -j16 FormatTests && ./tools/clang/unittests/Format/FormatTests

Reviewers: jolesiak, klimek

Reviewed By: jolesiak, klimek

Subscribers: klimek, cfe-commits, Wizard

Differential Revision: https://reviews.llvm.org/D47095

llvm-svn: 333553