stage2: improved union support

 * `Module.Union.getFullyQualifiedName` returns a sentinel-terminated
   slice so that backends that need null-termination do not need an
   additional copy.
 * Module.Union: implement a `getLayout` function which returns
   information about ABI size and alignment so that the LLVM backend can
   properly lower union types into llvm types.
 * Sema: `resolveType` now returns `error.GenericPoison` rather than a
   Type with tag `generic_poison`. Callsites that want to allow that
   need to bypass this higher-level function.
 * Sema: implement coercion of enums and enum literals to unions.
 * Sema: fix comptime mutation of pointers to unions
 * LLVM backend: fully implement proper lowering of union types and
   values according to the union layout, and update the handling of AIR
   instructions that deal with unions to support union layouts.
 * LLVM backend: handle `decl_ref_mut`
   - Maybe this should be unreachable since comptime vars should be
     changed to be non-mutable when they go out of scope, but it's
     harmless for the LLVM backend to support lowering the value.
 * Type: fix `requiresComptime` for optionals, pointers, and some other
   types. This function is still wrong for structs, unions, and enums.

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