[jdk11u] RFR: 8229258: Backport patch from JDK 14 to allow building with Clang 13

[Campbell Jones]

This patch backports changes from JDK14 commit ae5615c6142a4dc0d9033462f4880d7b3c127e26. It does not rename the class to markWord as the original patch does, but instead converts markOop to a markWord equivalent in-place. This patch allows the JDK to be built with Clang 13, as it avoids undefined behavior. 

See the following for more information:
- https://bugs.openjdk.java.net/browse/JDK-8229258
- https://bugs.gentoo.org/817995
- https://mail.openjdk.java.net/pipermail/hotspot-dev/2019-July/038704.html
- https://mail.openjdk.java.net/pipermail/hotspot-dev/2019-July/038712.html
- https://reviews.llvm.org/D99790#2678384

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Commit messages:
 - 8229258: Backport patch from JDK 14 to allow building with Clang 13

Changes: https://git.openjdk.java.net/jdk11u/pull/23/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u&pr=23&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8229258
  Stats: 788 lines in 103 files changed: 51 ins; 6 del; 731 mod
  Patch: https://git.openjdk.java.net/jdk11u/pull/23.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u pull/23/head:pull/23

PR: https://git.openjdk.java.net/jdk11u/pull/23