RFR: 8307629: FunctionDescriptor::toMethodType should allow sequence layouts (mainline)

[Maurizio Cimadamore]

This is a port of: https://git.openjdk.org/panama-foreign/pull/830

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jdk/pull/13869/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13869&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307629
  Stats: 75 lines in 5 files changed: 54 ins; 10 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/13869.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13869/head:pull/13869

PR: https://git.openjdk.org/jdk/pull/13869