* 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.
This adds basic support for calling the verifier on global declarations
that are expected to represent symbol exports. The driver now
exclusively uses this for knowing what symbols make up a TBD file.
Future patches will check against the dylib's symbol table.
