[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
12 files changed: