[analyzer] Always allow BugReporterVisitors to see the bug path.

[analyzer] Always allow BugReporterVisitors to see the bug path.

Before, PathDiagnosticConsumers that did not support actual path output
would (sensibly) cause the generation of the full path to be skipped.
However, BugReporterVisitors may want to see the path in order to mark a
BugReport as invalid.

Now, even for a path generation scheme of 'None' we will still create a
trimmed graph and walk backwards through the bug path, doing no work other
than passing the nodes to the BugReporterVisitors. This isn't cheap, but
it's necessary to properly do suppression when the first path consumer does
not support path notes.

In the future, we should try only generating the path and visitor-provided
path notes once, or at least only creating the trimmed graph once.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@164447 91177308-0d34-0410-b5e6-96231b3b80d8