RFR: 8214363: HeapWord should not be a fake class

[Kim Barrett]

I meant to send this to hotspot-dev, not hotspot-runtime-dev.  Please use the other thread.

> On Feb 14, 2019, at 5:04 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
> 
> Please review this change to the definitions of HeapWord and MetaWord.
> They are no longer a class type.  Rather, they are typedefs for a
> pointer to associated incomplete (and never defined) helper classes.
> Thus, we no longer pretend to have class objects where they never
> existed.
> 
> Among other things, this makes memset on a HeapWord[] no longer trigger
> gcc8's new -Wclass-memaccess in ObjectStartArray::set_covered_region().
> 
> Other than the change to the definitions of HeapWord and MetaWord,
> most of the changes are removal of no longer valid forward class
> declarations for those types.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8214363
> 
> Webrev:
> http://cr.openjdk.java.net/~kbarrett/8214363/open.02/
> 
> Testing:
> mach5 tier1-3.