[foreign-memaccess+abi] RFR: 8307253: Make FunctionDescriptor::toMethodType an instance method of Linker

[Maurizio Cimadamore]

This patch moves `FunctionDescriptor::toMethodType` as an instance method of `Linker`. This allows linker implementations to be more opinionated on how the function descriptor -> method type conversion should behave.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/830/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=830&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307253
  Stats: 141 lines in 9 files changed: 60 ins; 68 del; 13 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/830.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/830/head:pull/830

PR: https://git.openjdk.org/panama-foreign/pull/830