JSpecify: work around missing type annotations in cast expression types

[msridhar]

E.g., for this:

Object o = ...;
var x = (A<@Nullable String>) o;

The type of (A<@Nullable String>) o provided by javac is A<String>, not A<@Nullable String>. We should work around this.

[msridhar]

This issue is fixed in JDK 24-ea, but most likely we should still implement a workaround in NullAway.

[msridhar]

From my testing, it looks like this is also fixed in the latest JDK 23 release candidate. FYI @cpovirk