[foreign-memaccess+abi] RFR: 8294621: Copying from a StringCharBuffer-backed MemorySegment results in crash [v3]

[Per Minborg]

> This PR prevents `StringCharBuffer` arguments in `MemorySegment::ofBuffer`.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Update after comment

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/741/files
  - new: https://git.openjdk.org/panama-foreign/pull/741/files/b9114e07..f5f1ec47

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=741&range=02
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=741&range=01-02

  Stats: 3 lines in 3 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/741.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/741/head:pull/741

PR: https://git.openjdk.org/panama-foreign/pull/741