Check for unsafe constructs in all modes, so that we can emit the
"unsafe does not cover any unsafe constructs" warning consistently.
One does not need to write "unsafe" outside of strict memory safety
mode, but if you do... it needs to cover unsafe behavior.
Instead of diagnosing in CSApply, let's create a
fix and diagnose in the solver instead.
Additionally, make sure we assign ErrorTypes to
any VarDecls bound by the invalid pattern, which
fixes a crash.
rdar://110638279
