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

[Kim Barrett]

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

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

Commit messages:
 - Reimpliment PC::cast, with some cases now constexpr.

Changes: https://git.openjdk.java.net/jdk/pull/143/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=143&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8247909
  Stats: 237 lines in 3 files changed: 111 ins; 110 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/143.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/143/head:pull/143

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