x86_64: implement error set and enum safety

This is all of the expected 0.14.0 progress on #21530, which can now be
postponed once this commit is merged.

This required rewriting the (un)wrap operations since the original
implementations were extremely buggy.

Also adds an easy way to retrigger Sema OPV bugs so that I don't have to
keep updating #22419 all the time.

test/cases/compile_errors/reassign_to_array_parameter.zig

@@ -6,7 +6,7 @@ export fn entry() void {
 }
 
 // error
-// backend=llvm
+// backend=stage2
 // target=native
 //
 // :2:5: error: cannot assign to constant