[analyzer] Allow a BugReport to be marked "invalid" during path generation.

This is intended to allow visitors to make decisions about whether a
BugReport is likely a false positive. Currently there are no visitors
making use of this feature, so there are no tests.

When a BugReport is marked invalid, the invalidator must provide a key
that identifies the invaliation (intended to be the visitor type and a
context pointer of some kind). This allows us to reverse the decision
later on. Being able to reverse a decision about invalidation gives us more
flexibility, and allows us to formulate conditions like "this report is
invalid UNLESS the original argument is 'foo'". We can use this to
fine-tune our false-positive suppression (coming soon).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@164446 91177308-0d34-0410-b5e6-96231b3b80d8

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