git: openjdk/jdk16u: 8263968: CDS: java/lang/ModuleLayer.EMPTY_LAYER should be singleton

[Alexander Scherbatiy]

Changeset: 5375fda6
Author:    Alexander Scherbatiy <alexsch at openjdk.org>
Date:      2021-03-26 13:01:29 +0000
URL:       https://git.openjdk.java.net/jdk16u/commit/5375fda6

8263968: CDS: java/lang/ModuleLayer.EMPTY_LAYER should be singleton

Backport-of: 133a63b4a1a2ef2e4a51fdf9edac073078692f39

! src/hotspot/share/memory/heapShared.cpp
! src/java.base/share/classes/java/lang/ModuleLayer.java
! test/hotspot/jtreg/runtime/cds/appcds/cacheObject/CheckArchivedModuleApp.java