Add 'nopartial' qualifier for availability attributes.

An optional nopartial can be placed after the platform name.
int bar() __attribute__((availability(macosx,nopartial,introduced=10.12))

When deploying back to a platform version prior to when the declaration was
introduced, with 'nopartial', Clang emits an error specifying that the function
is not introduced yet; without 'nopartial', the behavior stays the same: the
declaration is `weakly linked`.

A member is added to the end of AttributeList to save the location of the
'nopartial' keyword. A bool member is added to AvailabilityAttr.

The diagnostics for 'nopartial' not-yet-introduced is handled in the same way as
we handle unavailable cases.

Reviewed by Doug Gregor and Jordan Rose.

rdar://23791325

llvm-svn: 261163