Change how we handle diagnose_if attributes.

This patch changes how we handle argument-dependent `diagnose_if`
attributes. In particular, we now check them in the same place that we
check for things like passing NULL to Nonnull args, etc. This is
basically better in every way than how we were handling them before. :)

This fixes PR31638, PR31639, and PR31640.

Differential Revision: https://reviews.llvm.org/D28889

llvm-svn: 293360