Reimpliment PrimitiveConversions::cast, with some cases now constexpr.
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### Progress
- [x] Change must not contain extraneous whitespace
- [x] Commit message must refer to an issue
- [ ] Change must be properly reviewed
### Issue
* [JDK-8247909](https://bugs.openjdk.java.net/browse/JDK-8247909): Improve PrimitiveConversions::cast using C++14
### Download
`$ git fetch https://git.openjdk.java.net/jdk pull/143/head:pull/143`
`$ git checkout pull/143`
Reviewed-by: dholmes, stefank
7eb4d4aa01