This enhancement provides a way to specify more granular levels for the GC verification enabled using the "VerifyBeforeGC", "VerifyAfterGC" and "VerifyDuringGC" diagnostic options. It introduces a new diagnostic option VerifySubSet using which one can specify the subset of the memory system that should be verified.

With this new option, one or more sub-systems can be specified in a comma separated string. Valid memory sub-systems are: threads, heap, symbol_table, string_table, codecache, dictionary, classloader_data_graph, metaspace, jni_handles, c-heap and codecache_oops.
 
During the GC verification, only the sub-systems specified using VerifySubSet get verified:
 ```
D:\tests>java -XX:+UnlockDiagnosticVMOptions -XX:+VerifyBeforeGC -XX:VerifySubSet="threads,c-heap" -Xlog:gc+verify=debug Test
[0.095s][debug ][gc,verify] Threads
[0.099s][debug ][gc,verify] C-heap
[0.105s][info ][gc,verify] Verifying Before GC (0.095s, 0.105s) 10.751ms
[0.120s][debug ][gc,verify] Threads
[0.124s][debug ][gc,verify] C-heap
[0.130s][info ][gc,verify] Verifying Before GC (0.120s, 0.130s) 9.951ms
[0.148s][debug ][gc,verify] Threads
[0.152s][debug ][gc,verify] C-heap
```
If any invalid memory sub-systems are specified with VerifySubSet, Java process exits with the following error message:
```
D:\tests>java -XX:+UnlockDiagnosticVMOptions -XX:+VerifyBeforeGC -XX:VerifySubSet="threads,c-heap,hello" -Xlog:gc+verify=debug oom
Error occurred during initialization of VM
VerifySubSet: 'hello' memory sub-system is unknown, please correct it
```