RFR: 8214363: HeapWord should not be a fake class

[Kim Barrett]

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most of the changes are removal of no longer valid forward class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.
Built using gcc8.2 and verified there is no longer a -Wclass-memaccess
warning from objectStartArray.cpp.