Remove [special] entry for ucontext.h.

Since it turns out that OSX isn't completely unilaterally deprecating
ucontext.h -- just jumping the gun; it's marked obsolescent in POSIX-2008,
which OSX10.6 doesn't support, but whatever -- remove the [special] entry
for it and treat it as another known-and-uninteresting ann_buggy case.
Recognize Apple clang.  Record compiler diagnostics when a file fails the 'baseline' test.
Gen3 inventories for OSX 10.6.  Various adjustments to the decltests required.