There were two issues with Objective-C XCTest discovery:
1. We were relying on syntactic test discovery after a document is edited. But since we don't have syntactic test discovery for Objective-C tests, this meant that all tests would disappear as a document got edited. Until we get syntactic test discovery for Objective-C, use the semantic index to discover tests, even if they are out-of-date.
2. We were assuming that the `DocumentSymbols` request returned `[DocumentSymbol]` to find the ranges of tests. But clangd returns the alternative `[SymbolInformation]`, which meant that we were only returning the position of a test function’s name instead of the test function’s range.
rdar://126810202
When the semantic index is out-of-date, we currently purely rely on the syntactic index to discover tests and completely ignore data from the semantic index. This may lead to confusing behavior. For example if you have
```
class MightInheritFromXCTestCaseOrNot {}
class MyClass: MightInheritFromXCTestCaseOrNot {
func testStuff() {}
}
```
Then we don’t return any tests when the semantic index is up-to-date. But once the file is modified (either on disk or in-memory), we purely rely on the syntactic index, which reports `testStuff` as a test method. After a build / background indexing finishes, the test method disappears again.
We can mitigate this problem as follows: If we have stale semantic index data for the test file, for every test method found by the syntactic index, check if we have an entry for this method in the semantic index. If we do, but that entry is not marked as a test class/method, we know that the semantic index knows about this method but decided that it’s not a test method for some reason. So we should ignore it.
rdar://126492948
If the index for a given file is not up-to-date, perform a syntactic scan for tests within it.
This should allow editors to run the `textDocument/tests` request while the user is editing a file to scan it for test cases.
The naming was quite inconsistent here. Let’s rename these to `LanguageService` to highlight that they belong together.
- ToolchainLanguageServer -> LanguageService
- SwiftLanguageServer -> SwiftLanguageService
- ClangLanguageServerShim -> ClangLanguageService
