[SSAF] Properly handle contributors with multiple declarations (#204482)
A contributor entity can have multiple declarations all contributing
interesting facts. For example, a function declaration (not definition)
may have default arguments, which may provide pointer flow or unsafe
buffer usage facts. This commit groups declarations by their canonical
decls. The entity summary of a contributor will be collected from all
its decls.
In addition, this commit includes the following minor changes:
- Factor the common procedure of summary extraction and insertion into a
template function in SSAFAnalysesCommon.h.
- Convert the no-duplicate contributor assertion into a debug warning.
We need the release build to not crash.
rdar://179150798