RFR: Make attributes mutable [v3]

[Maurizio Cimadamore]

> This PR makes the attribute list associated with a declaration (and a type) mutable. This means that we can now tweak a declaration/type after the fact. While the declaration/type is still immutable, the information attached to it can grow over time. This should be useful to reduce coupling between different steps in the extraction process. I plan to use this to simplify the way we deal with name mangling.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Make addAddtribute idempotent
  Add some comments on the internal attributes
  Tweak EnumConstant attribute to contain the name of the parent enum (to avoid circular recusion when printing a declaration)

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Changes:
  - all: https://git.openjdk.org/jextract/pull/146/files
  - new: https://git.openjdk.org/jextract/pull/146/files/a02f1e7c..fbc41980

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=146&range=02
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=146&range=01-02

  Stats: 14 lines in 4 files changed: 8 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/jextract/pull/146.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/146/head:pull/146

PR: https://git.openjdk.org/jextract/pull/146