[lworld] RFR: 8341771: [lworld] Heap dump recognizing new flat fields formats [v2]

[Alex Menkov]

> The fix re-implements flat object support in heap dumper.
> New approach does not require changes in .hprof format.
> Flat value objects are dumped as heap-allocated objects with generated ID.
> For heap-allocated objects the ID is their address (oop), they are always aligned.
> ID generator for flat objects uses unaligned values (so there is no conflicts with oops).
> 
> HeapDump test was reimplemented (it was disabled for a long time)
> Changes in hprof test lib were reverted (they are not needed anymore)
> 
> testing: tier1..4,hs-tier5-svc

Alex Menkov has updated the pull request incrementally with one additional commit since the last revision:

  removed T_FLAT_ELEMENT in StackRefDumper

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

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1700/files
  - new: https://git.openjdk.org/valhalla/pull/1700/files/796bf148..a980b464

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1700&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1700&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/1700.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1700/head:pull/1700

PR: https://git.openjdk.org/valhalla/pull/1700