RFR: 7903573: Jextract does not support atomic types

[Maurizio Cimadamore]

This PR fixes a crash when jextract processes atomic types. These types are exposed in libclang in a weird way: their kind is set to `Atomic` but they do not reveal what the underlying type is (except for the type spelling). The underlying type has to be discovered using the `clang_Type_getValueType` function on the `Atomic` type.

This PR does just that, and also adds a test for various atomic types.

-------------

Commit messages:
 - Remove whitespaces
 - Add copyright
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/132/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=132&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903573
  Stats: 158 lines in 6 files changed: 158 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jextract/pull/132.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/132/head:pull/132

PR: https://git.openjdk.org/jextract/pull/132