RFR: 8310948: Fix ignored-qualifiers warning in Hotspot

[Daniel Jeliński]

Please review this attempt to fix ignored-qualifiers warning.

Example warnings:

src/hotspot/share/oops/method.hpp:413:19: warning: 'volatile' type qualifier on return type has no effect [-Wignored-qualifiers]
   CompiledMethod* volatile code() const;
                   ^~~~~~~~~


src/hotspot/share/jfr/periodic/jfrModuleEvent.cpp:65:20: warning: type qualifiers ignored on cast result type [-Wignored-qualifiers]
    65 |   event.set_source((const ModuleEntry* const)from_module);
       |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


The proposed fix removes the ignored qualifiers.
In a few AD files I replaced `const` with `constexpr` where I noticed that the method is returning a compile-time constant, and other platforms use `constexpr` on the same method.

Release, debug and slowdebug builds on Aarch64 / x64 and Mac / Linux complete without errors. Cross-compile GHA builds also pass.

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

Commit messages:
 - Fix other platforms
 - Fix shenandoah
 - Fix ignored-qualifiers warning

Changes: https://git.openjdk.org/jdk/pull/14674/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14674&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310948
  Stats: 223 lines in 74 files changed: 0 ins; 0 del; 223 mod
  Patch: https://git.openjdk.org/jdk/pull/14674.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14674/head:pull/14674

PR: https://git.openjdk.org/jdk/pull/14674