[foreign-memaccess+abi] RFR: 8300294: Add tests for by-value unions and structs with nested fixed-length arrays [v2]

[Jorn Vernee]

> I've added some tests for by-value unions, structs/unions nested into other structs, and aggregates with nested inline arrays.
> 
> These test cases are taken from a fuzzer I wrote a while ago. So, they are a bit random in terms of the generated layouts, but should nonetheless  serve as a basic test for the cases that we miss with TestDowncall/TestUpcall today. This mostly helps give some coverage of the linker's classification code for these cases.
> 
> I've also pulled in some test value generation code, which can generate random test values when given a MemoryLayout, allocator and random generator. (if it seems useful, I can re-write the existing tests to use this generation code as well, but I'd like to save that for a followup PR).
> 
> Unfortunately, the fallback linker doesn't seem to be able to reliably handle by-value unions. It's a know issue with libffi. I've filed: https://bugs.openjdk.org/browse/JDK-8301800 for now. I've changed the implementation to throw an IAE, which was already specified for `Linker::downcallHandle/upcallStub`: 
> 
> https://github.com/openjdk/panama-foreign/blob/f61f3a31af4976d0e64d3bfa72cda95b501e2a7d/src/java.base/share/classes/java/lang/foreign/Linker.java#L232-L235
> 
> There was also a bug in the classification of HFAs on AArch64 which I've fixed. The issue was that structs with float/double fields that are nested into arrays or other structs/unions should also be counted as HFAs. But, the correct classification logic only accounted for the 'flat' case, where a struct only contains ValueLayouts.

Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:

  review comments

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

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/780/files
  - new: https://git.openjdk.org/panama-foreign/pull/780/files/c47df9ed..944dfd1e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=780&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=780&range=00-01

  Stats: 14 lines in 2 files changed: 8 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/780.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/780/head:pull/780

PR: https://git.openjdk.org/panama-foreign/pull/780