[ThinLTO] Ignore callee edge to global variable

Since the symbols in the ThinLTO summary are indexed by GUID we can end
up in corner cases where a callee edge in the combined index goes to a
summary for a global variable. This could happen in the case of hash
collisions, and in the case of SamplePGO profiles could potentially happen
due to code changes (since we synthesize call edges to GUIDs that were
inlined callees in the profiled code).

Handle this by simply ignoring any non-FunctionSummary callees.

Differential Revision: https://reviews.llvm.org/D152406