Eg. when requesting type hierarchy of a class when the project hasn’t been built, sourcekit-lsp returns an empty array. That causes VS Code to fail with very ambiguous error messages
- MISSING provider
- Cannot read properties of null (reading 'kind')
To work around this, instead of returning an empty array, return `nil` instead.
rdar://126228814
Eg. if we have the following, and we get the call hierarchy of `foo`, we only want to show `bar` once, with multiple `fromRanges` instead of having two entries for `bar` in the call hierarchy.
```swift
func foo() {}
func bar() {
foo()
foo()
}
```
This ways the client doesn’t need to create a hierarchical structure using the container names. It is also more flexible and allows nesting of test suites + the addition of labels and tags for swift-testing.
The data structure for `TestItem` has been heavily inspired by VS Code’s `TestItem` for the test explorer, which should make it fairly straightforward to integrate these results into the VS Code test explorer.
The naming was quite inconsistent here. Let’s rename these to `LanguageService` to highlight that they belong together.
- ToolchainLanguageServer -> LanguageService
- SwiftLanguageServer -> SwiftLanguageService
- ClangLanguageServerShim -> ClangLanguageService
