RFR: 8247909: Improve PrimitiveConversions::cast using C++14

[David Holmes]

On Sun, 13 Sep 2020 07:27:13 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this reimplementation of PrimitiveConversions::cast,
> using facilities from C++14.  In particular, conversions involving
> only integral values or integral and enum values are now constexpr.
> Also using <type_traits> rather than HotSpot work-alikes.
> 
> Had to update zForwardingEntry.hpp because of a class no longer being
> implicitly included.  Improved a friend declaration while in the
> vicinity.
> 
> Testing
> mach5 tier1

Marked as reviewed by dholmes (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/143