stage2: implement runtime `@intToEnum`

 * Update AIR instruction `intcast` to allow the dest type to be an
   enum.
 * LLVM backend: update `intcast` to support when the bit counts of
   operand and dest type are the same. This was already a requirement of
   the instruction previously.
 * Type: `intInfo` supports the case when the type is an enum, and
   retrieves the info for the integer tag type. This makes it pretty
   easy for backends to implement `intcast` without having to care
   explicitly that the new type is an enum. As a bonus, simple enums
   never have to go through the type system; their signedness and bit
   count are computed directly.

The "int to enum" behavior test case is now passing for stage2 in the
LLVM backend.

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