Fix a crash when re-hashing the basic-block map would leave behind a dangling
mutable reference.
The irony that the language feature this pass implements would have caught this
issue statically is not lost on me.
Add a diagnostic pass that emits errors when a violation of the "Law of
Exclusivity" is detected at compile time. The Law of Exclusivity requires
that the access duration of any access to an address not overlap
with an access to the same address unless both accesses are reads.
This pass relies on 'begin_access' and 'end_access' SIL instruction
markers inserted by SILGen to determine when an access to an address begins and
ends. It models the in-progress accesses with a map from storage locations to
the counts of read and write-like accesses in progress for that location.
