Revert r298491 and r298494 which changed Clang's handling of 'nonnull'

attributes.

These patches don't work because we can't currently access the parameter
information in a reliable way when building attributes. I thought this
would be relatively straightforward to fix, but it seems not to be the
case. Fixing this will requrie a substantial re-plumbing of machinery to
allow attributes to be handled in this location, and several other fixes
to the attribute machinery should probably be made at the same time. All
of this will make the patch .... substantially more complicated.

Reverting for now as there are active miscompiles caused by the current
version.

llvm-svn: 298695