@sizeOf returns 0 for comptime types

This defines `@sizeOf` to be the runtime size of a type, which means
that it is zero for types such as comptime_int, type, and (enum
literal).

See #2209

test/compile_errors.zig

@@ -2,6 +2,15 @@ const tests = @import("tests.zig");
 const builtin = @import("builtin");
 
 pub fn addCases(cases: *tests.CompileErrorContext) void {
+    cases.add(
+        "@sizeOf bad type",
+        \\export fn entry() void {
+        \\    _ = @sizeOf(@typeOf(null));
+        \\}
+    ,
+        "tmp.zig:2:17: error: no size available for type '(null)'",
+    );
+
     cases.add(
         "Generic function where return type is self-referenced",
         \\fn Foo(comptime T: type) Foo(T) {