Sema now properly handles alloc_inferred and alloc_inferred_mut ZIR
instructions inside a comptime execution context. In this case it
creates Decl objects and points to them with the new `decl_ref_mut`
Value Tag. `storePtr` is updated to mutate such Decl types and values.
In this case it destroys the old arena and makes a new one, preventing
memory growth during comptime code execution.
Additionally:
* Fix `storePtr` to emit a compile error for a pointer comptime-known
to be undefined.
* Fix `storePtr` to emit runtime instructions for all the cases that a
pointer is comptime-known but does not support comptime
dereferencing, such as `@intToPtr` on a hard-coded address, or an
extern function.
* Fix `ret_coerce` not coercing inside inline function call context.
