[foreign] RFR 8220063: Generate LayouType constants as part of jextract output

[Jorn Vernee]

Hi,

I'd like to contribute a patch which let's jextract generate LayoutType 
constants in typedef annotation classes, and struct classes for the 
underlying type. The LayoutType constant has the name of the typedef or 
struct with an "_t" suffix.

This is useful because;

1.) No need to manually declare a static final LayoutType if you don't 
want to call LayoutType.ofStruct(...) everywhere.

2.) If an API uses a type in it's documentation/tutorials, but that type 
comes from a typedef, in the generated Jextract bindings you will just 
find the typedef annotation with that name. But, if we add a LayoutType 
constant to the typedef annotation class, the user has an easy way to 
find the underlying type as well. (Very useful with APIs that use 
typedefs for pretty much anything. Like the Windows API).

I might be a little biased towards this, but I've found that declaring 
static final LayoutType constants is a must have in non-trivial 
projects. Both to reduce verbosity and to show the name of the API type 
in the code, not just the name of the underlying type. e.g. 
`scope.allocate(MyType_t)` vs. `scope.allocate(DOUBLE.array(2))`.

Bug: https://bugs.openjdk.java.net/browse/JDK-8220063
Webrev: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/8220063/webrev.00/

I did 3 other things to implement this:

1.) Drop Address parameter of LayoutType.ofFunction, since we can just 
infer that from the @NativeCallback annotation instead.
2.) Fixed issue where a typedef of a function type was not generating an 
@NativeCallback class.
3.) Sharpened typing in TypeDictionary and JType w.r.t. 
JType.FunctionalInterfaceType.

Thanks,
Jorn