Many clients of the conformance lookup operations would prefer to get
an invalid conformance (== there is no conformance) rather than a
missing conformance. Parameterize the conformance lookup operations so
that most callers won't see missing conformances, by filtering them
out at the end. Opt-in those callers that do want to see missing
conformances so they can be diagnosed.
Adds a new 'key.retrieve_symbol_graph' option to the request. When set to 1 it
includes the JSON for a SymbolGraph containing a single node for the symbol at
the requested position.
This also extends the SymbolGraph library with a new entry point to get a graph
for a single symbol, and to additionally support type substitution to match the
existing CursorInfo behavior (e.g. so that when invoked on `first` in
`Array<Int>().first`, the type is given as `Int?` rather than `Element?`).
Resolves rdar://problem/70551509
