RFR: 8343513: Forward declare Thread in mutexLocker.hpp [v3]

[Johan Sjölen]

> Hi,
> 
> Let's explicitly forward declare `Thread` in `mutexLocker.hpp` instead of including `thread.hpp`. Previously we got the forward declaration from `memory/allocation.hpp`, but let's be explicit about forward declarations being used when we can.
> 
> Thanks

Johan Sjölen has updated the pull request incrementally with one additional commit since the last revision:

  Typo

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21868/files
  - new: https://git.openjdk.org/jdk/pull/21868/files/b45dc667..6057ff6c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21868&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21868&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21868.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21868/head:pull/21868

PR: https://git.openjdk.org/jdk/pull/21868