Downgrade complaints about calling unavailable functions to a warning

(as GCC does), except when we've performed overload resolution and
found an unavailable function: in this case, we actually error.

Merge the checking of unavailable functions with the checking for
deprecated functions. This unifies a bit of code, and makes sure that
we're checking for unavailable functions in the right places. Also,
this check can cause an error. We may, eventually, want an option to
make "unavailable" warnings into errors.

Implement much of the logic needed for C++0x deleted functions, which
are effectively the same as "unavailable" functions (but always cause
an error when referenced). However, we don't have the syntax to
specify deleted functions yet :)

llvm-svn: 64955