* This completes support for verifying every declaration found in a
header is discovered in the dylib. Diagnostics are reported for each
class for differences that are representable in TBD files.
* This patch also now captures unavailable attributes that depend on
target triples. This is needed for proper tbd file generation.
Previously this class was only used by ExtractAPI, but it will soon also
be needed by InstallAPI. This patch should not change availability
behavior but just centralizes the information next to what already is
captured about availability for AST traversal.