RFR: 8328235: GenShen: Robustify ShenandoahGCSession and fix missing use

[Y. Srinivas Ramakrishna]

ShenandoahGCSession is intended to create a scope where the ShenandoahHeap's _gc_cause and _gc_generation field reflect the current gc cycle. We now check that we do not overwrite existing non-default settings (respectively _no_gc and nullptr). The destructor of the scope/stack object also resets these fields to their default settings, ensuring intended uses. This uncovered a situation where the scope was not entered when it should have been, which we have now fixed.

Testing in progress.

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Commit messages:
 - Robustify ShenandoahGCSession, and fix a missing use for

Changes: https://git.openjdk.org/shenandoah/pull/407/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=407&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328235
  Stats: 13 lines in 2 files changed: 13 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/407.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/407/head:pull/407

PR: https://git.openjdk.org/shenandoah/pull/407