[SSAF][Extractor] Make hard errors in PointerFlow and UnsafeBufferUsage Extractors quiet (#201953)
Hard errors were used in extractors during development to quickly
identify unsupported language constructs. This commit converts them to
DEBUG_WITH_TYPE so that these errors are silenced in release builds.
In addition, translating unsupported language constructs now silently
results in an empty EntityPointerLevelSet. The PointerFlowExtractor will
skip empty sets for either the source or the destination when building
edges to avoid an ill-formed edge set data structure.
rdar://178747892
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Co-authored-by: Balázs Benics <benicsbalazs@gmail.com>