RFR: 7903605: Simplify TypeMaker

[Maurizio Cimadamore]

The main goal of this PR is to make TypeMaker simpler, and remove the convoluted logic which stashes pointers, and asks client to call a method to resolve their types.

Instead, this patch tweaks the behavior of the TypeMaker so that when a pointer to a declared type is created, it points to a special supplier. This supplier, when called, looks in the declaration cache for a matching declaration (in the `TreeMaker`). If a corresponding declaration is found, a new declared type pointing to that declaration is returned as a pointee type.

It is also possible for a pointer to point to an opaque struct, in which case there might be no symbol in the declaration cache. In such case we make the supplier return an erroneous type instead.

The patch also addresses a number of related issues:

* Since declarations are stable identity-wise but types are not, I removed the `Attributed` interface and corresponding implementation class. Now all the methods to deal with attributes are in `Declaration` - declarations are the only entities that can be reliably attributed (because of de-duplication).

* I added a mechanism to create "nested" `TreeMaker` - that is, a `TreeMaker` that can delegate a symbol lookup to a parent `TreeMaker`. This is handy when dealing with macros, where we do not want to pollute the original environment, but we still want to be able to resolve declarations included in that environment.

* I have fixed the equals methods in Declaration - these methods ignored attributes and were not working correctly. As a result, I also did some changes in duplicate filter, as using declaration equality does not work (because attributes might mismatch thanks to the added `Skip`).

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Commit messages:
 - Fix support for pointers to opaque types
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/155/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=155&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903605
  Stats: 514 lines in 14 files changed: 114 ins; 297 del; 103 mod
  Patch: https://git.openjdk.org/jextract/pull/155.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/155/head:pull/155

PR: https://git.openjdk.org/jextract/pull/155