stage2: change @bitCast to always be by-value

After a discussion about language specs, this seems like the best way to
go, because it's simpler to reason about both for humans and compilers.

The `bitcast_result_ptr` ZIR instruction is no longer needed.

This commit also implements writing enums, arrays, and vectors to
virtual memory at compile-time.

This unlocked some more of compiler-rt being able to build, which
in turn unlocks saturating arithmetic behavior tests.

There was also a memory leak in the comptime closure system which is now
fixed.

src/Module.zig

@@ -317,6 +317,7 @@ pub const WipCaptureScope = struct {
         assert(!self.finalized);
         // use a temp to avoid unintentional aliasing due to RLS
         const tmp = try self.scope.captures.clone(self.perm_arena);
+        self.scope.captures.deinit(self.gpa);
         self.scope.captures = tmp;
         self.finalized = true;
     }