The switch_br ZIR instructions are now switch_block instructions. This
avoids a pointless block always surrounding a switchbr in emitted ZIR
code.

Introduce typeof_elem ZIR instruction for getting the type of the
element of a pointer value in 1 instruction.

Change typeof to be un_node, not un_tok.

Introduce switch_capture ZIR instructions for obtaining the capture
value of switch prongs.

Introduce Sema.resolveBody for when you want to extract a *Inst out of a
block and you know that there is only going to be 1 break from it.

What's not working yet: AstGen does not correctly elide
store instructions when it turns out that the result location does not
need to be used as a pointer.

Also Sema validation code for duplicate switch items is not yet
implemented.