[jdk11u-dev] RFR: Allow building with Clang 13 [v2]

[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.

Salander has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains one commit:

  Allow building with Clang 13
  
  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 here: https://bugs.openjdk.java.net/browse/JDK-8229258

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

Changes: https://git.openjdk.org/jdk11u-dev/pull/1284/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1284&range=01
  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