RFR: 8299673: Simplify object pinning interactions with string deduplication

[Erik Österlund]

When raw char* String contents are exposed to JNI code, we 

1. Load the string.value and pin it 
2. Run native code 
3. Load the string.value and unpin it 

Given this sequence we would be in trouble if between 1 and 3, string deduplication changed the value object. Then the pinning and unpinning wouldn't be balanced. 

The current approach for dealing with this is to have a bunch of code to guard against deduplication. An alternative simpler solution is to just change step 3 to pass in the same value. We already have enough information available to do that. Then the pinning and unpinning is also balanced, and we don't need to have any special interactions with string deduplication and can decouple these orthogonal concerns.

It's worth noting though that the contract of pin_object now makes it explicit that pinned objects must not be recycled, even if not otherwise reachable. That seems to come naturally for region based pinning, but is worth keeping in mind. The exposed char* might be the only thing referencing the string value when string dedup happens concurrently.

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Commit messages:
 - More Kim feedback
 - Feedback from Kim
 - 8299673: Simplify object pinning interactions with string deduplication

Changes: https://git.openjdk.org/jdk/pull/11923/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11923&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299673
  Stats: 162 lines in 14 files changed: 66 ins; 68 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/11923.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11923/head:pull/11923

PR: https://git.openjdk.org/jdk/pull/11923