stage2: implement switching on unions

* AstGen: Move `refToIndex` and `indexToRef` to Zir
 * ZIR: the switch_block_*_* instruction tags are collapsed into one
   switch_block tag which uses 4 bits for flags, and reduces the
   scalar_cases_len field from 32 to 28 bits.
   This freed up more ZIR tags, 2 of which are now used for
   `switch_cond` and `switch_cond_ref` for producing the switch
   condition value. For example, for union values it returns the
   corresponding enum value.
 * switching with multiple cases and ranges is not yet supported because
   I want to change the ZIR encoding to store index pointers into the
   extra array rather than storing prong indexes. This will avoid O(N^2)
   iteration over prongs.
 * AstGen now adds a `switch_cond` on the operand and then passes the
   result of that to the `switch_block` instruction.
 * Sema: partially implement `switch_capture_*` instructions.
 * Sema: `unionToTag` notices if the enum type has only one possible value.

src/value.zig

@@ -116,7 +116,7 @@ pub const Value = extern union {
         decl_ref_mut,
         /// Pointer to a specific element of an array.
         elem_ptr,
-        /// Pointer to a specific field of a struct.
+        /// Pointer to a specific field of a struct or union.
         field_ptr,
         /// A slice of u8 whose memory is managed externally.
         bytes,