[jdk11u-dev] RFR: Allow building with Clang 13 [v9]

[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 incrementally with two additional commits since the last revision:

 - Fix github action
 - Temporarily build linux with clang as well for testing

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

Changes:
  - all: https://git.openjdk.org/jdk11u-dev/pull/1284/files
  - new: https://git.openjdk.org/jdk11u-dev/pull/1284/files/7578a4b5..dc8bedce

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1284&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1284&range=07-08

  Stats: 367 lines in 1 file changed: 367 ins; 0 del; 0 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