Use Class.getDeclaredField instead of Class.getField

Hi,

I encountered that issue with v3 but I believe the problem exists also in v4.

Basically in https://github.com/antlr/stringtemplate4/blob/9a439491acc5b17d191316c9b3a99ab7bd340477/src/org/stringtemplate/v4/misc/ObjectModelAdaptor.java#L150 you call `Class.getField` and then try to make it visible via `Field.setAccessible`, but this is incorrect by design as `Class.getField` only return public fields!

Instead you should be using `Class.getDeclaredField`.

In practice, it means that it is always impossible to access private fields from templates.

In v3, the issue was in `ASTExpr` but I guess this won't be fixed there? :)