This template function casts the result of `getValue()` or
`getStorageLocation()` to a given subclass of `Value` or
`StorageLocation` (using `cast_or_null`).
It's a common pattern to do something like this:
```cxx
auto *Val = cast_or_null<PointerValue>(Env.getValue(E));
```
This can now be expressed more concisely like this:
```cxx
auto *Val = Env.get<PointerValue>(E);
```
Instead of adding a new method `get()`, I had originally considered
simply adding a template parameter to `getValue()` and
`getStorageLocation()` (with a default argument of `Value` or
`StorageLocation`), but this results in an undesirable repetition at the
callsite, e.g. `getStorageLocation<RecordStorageLocation>(...)`. The
`Value` and `StorageLocation` in the method name adds nothing of value
when the template argument already contains this information, so it
seemed best to shorten the method name to simply `get()`.
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