[foreign-memaccess+abi] RFR: 8292037: Consider moving Linker::methodType as FunctionDescriptor::toMethodType

[Jorn Vernee]

This patch moves `Linker::methodType` to `FunctionDescriptor` as `carrierMethodType`.

I went with the name `carrierMethodType` instead of `toMethodType` (as suggested by the JBS issue) since I think it's a bit more descriptive. Please let me know your thoughts.

This issue also suggest adding support for `SequenceLayout` in the linker implementation. I think adding support in FunctionDescriptor could be a good idea, but since passing arrays by value is not supported in C, I feel like we should reject it in the linker implementations we have (which are all C as well). Of course, we could make it work any way by interpreting such cases as a pointer layout instead, but it's a case of being able to do the same thing in multiple ways (which I think can be confusing), and might actually be surprising if someone passes the wrong layout to the linker by accident, but it doesn't get "caught".

I've added support for `SequenceLayout` to `FunctionDescriptor`, but it is still rejected in the type classification logic.

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Commit messages:
 - fix javadoc
 - add support for SequenceLayout carriers in FunctionDescriptor only
 - Move Linker.methodType to FunctionDescriptor.carrierMethodType

Changes: https://git.openjdk.org/panama-foreign/pull/717/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=717&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292037
  Stats: 82 lines in 11 files changed: 38 ins; 31 del; 13 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/717.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/717/head:pull/717

PR: https://git.openjdk.org/panama-foreign/pull/717