Implement syntactic test scanning in clangd

[ahoppen]

Test discovery in SourceKit-LSP now relies on being able to do syntactic test discovery if the semantic index is not up-to-date. For Swift files, this is done using a SwiftSyntax scanner. SKAgent has the fast scanner through libclang API. We need to have an equivalent function in clangd that returns the syntactically discovered tests in a file (or probably multiple files so we can send batch queries for workspace-wide syntactic test discovery).

rdar://126492635