[clangd] Support `#pragma mark` in the outline

Xcode uses `#pragma mark -` to draw a divider in the outline view
and `#pragma mark Note` to add `Note` in the outline view. For more
information, see https://nshipster.com/pragma/.

Since the LSP spec doesn't contain dividers for the symbol outline,
instead we treat `#pragma mark -` as a group with children - the
decls that come after it, implicitly terminating when the symbol's
parent ends.

The following code:

```
@implementation MyClass

- (id)init {}

- (int)foo;
@end
```

Would give an outline like

```
MyClass
        > Overrides
                    > init
        > Public Accessors
                    > foo
```

Differential Revision: https://reviews.llvm.org/D105904