The underlying C++ code expects a non-null `Instruction*` or `SILArgument*` pointer, and
most of the contextual information in a verifier error is derived from these arguments,
so it doesn't really make sense for the Swift level interface to present these arguments
as optional.
This also required me to change how we handled which instruction/argument we
emit an error about in the verifier. Previously we were using two global
variables that we made nullptr to control which thing we emitted an error about.
This was unnecessary. Instead I added a little helper struct that internally
controls what we will emit an error about and an external "guard" RAII struct
that makes sure we push/pop the instruction/argument we are erroring upon
correctly.
