[clang][ExtractAPI] Add targetFallback to relationships in symbol graph

Adds a 'targetFallback' field to relationships in symbol graph that
contains the plain name of the relationship target. This is useful for
clients when the relationship target symbol is not available.

Differential Revision: https://reviews.llvm.org/D136455

clang/lib/ExtractAPI/Serialization/SymbolGraphSerializer.cpp

@@ -559,6 +559,7 @@ void SymbolGraphSerializer::serializeRelationship(RelationshipKind Kind,
   Object Relationship;
   Relationship["source"] = Source.USR;
   Relationship["target"] = Target.USR;
+  Relationship["targetFallback"] = Target.Name;
   Relationship["kind"] = getRelationshipString(Kind);
 
   Relationships.emplace_back(std::move(Relationship));