Add computed properties to `SymbolLocation` that centralise the
index-to-LSP coordinate conversion:
- `uri: DocumentURI?` — returns nil when `path` is empty.
- `lspPosition: Position` — converts the 1-based line/utf8Column to a
0-based LSP Position, using UTF-8 column as a UTF-16 approximation.
- `lspLocation: Location?` — wraps `documentUri` + `lspPosition` into
an LSP Location, returning nil when `path` is empty.
Update all call sites.
This cleanly separates the responsibilities for handling documentation from those of handling Swift files. It also simplifies providing docc support for clang because we just need to implement the two `symbolGraph` methods in `ClangLanguageService` and can re-use the remaining infrastructure from `DoccLanguageService`.
Rintaro Ishizaki
Anthony Latsis
Owen Voorhees
Matthew Bastien
Alex Hoppen