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

[Salander]

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

This is a re-do of @Serebit's PR that erroneously targeted the jdk11u repository ([link](https://github.com/openjdk/jdk11u/pull/23)), the only difference is that some errors in the backport were corrected in the `shenandoahForwarding.inline.hpp` file to fix some compilation errors when compiling with `shenandoahgc` enabled.

-------------

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

Changes: https://git.openjdk.org/jdk11u-dev/pull/1284/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1284&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8229258
  Stats: 788 lines in 103 files changed: 51 ins; 6 del; 731 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1284.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev pull/1284/head:pull/1284

PR: https://git.openjdk.org/jdk11u-dev/pull/1284