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
These symbols are understood to not be used for client API consumption
by convention so they should not appear in the generated symbol graph.
Differential Revision: https://reviews.llvm.org/D125678
