[analyzer] De-duplicate path diagnostics for each exploded graph node.

[analyzer] De-duplicate path diagnostics for each exploded graph node.

The bugreporter::trackNullOrUndefValue() mechanism contains a system of bug
reporter visitors that recursively call each other in order to track where a
null or undefined value came from, where each visitor represents a particular
tracking mechanism (track how the value was stored, track how the value was
returned from a function, track how the value was constrained to null, etc.).

Each visitor is only added once per value it needs to track. Almost. One
exception from this rule would be FindLastStoreBRVisitor that has two operation
modes: it contains a flag that indicates whether null stored values should be
suppressed. Two instances of FindLastStoreBRVisitor with different values of
this flag are considered to be different visitors, so they can be added twice
and produce the same diagnostic twice. This was indeed the case in the affected
test.

With the current logic of this whole machinery, such duplication seems
unavoidable. We should be able to safely add visitors with different flag
values without constructing duplicate diagnostic pieces. Hence the effort
in this commit to de-duplicate diagnostics regardless of what visitors
have produced them.

Differential Revision: https://reviews.llvm.org/D41258

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@321135 91177308-0d34-0410-b5e6-96231b3b80d8