Currently, `indexBasedDefinition` relies heavily on IndexStoreDB. If a symbol
belongs to a binary framework or a library that hasn't been indexed (but has
module info provided by sourcekitd), the definition request fails or returns
empty results.
This change adds a fallback mechanism in `definitionLocations`. When no
occurrences are found in the index, we check if `systemModule` information
is available on the symbol. If so, we trigger `definitionInInterface` to
generate the textual interface (via `editor.open.interface`) and return that
location.
This improves navigation for binary dependencies (XCFrameworks) and SDKs
partially covered by the index.
c40476c191