GC coverage can now occur in scenarios where a thread doesn't exist. (#33615)

* GC coverage can now occur in scenarios where a thread doesn't exist.
Properly handle this case and assert some invariant.

* Update gccover.cpp

Misspelled function referenced in comment.

* Remove MethodDesc from GC coverage.

* Code review and fix some asserts when running the Debug version of tests.

* Limit JIT flag manipulation to FEATURE_TIERED_COMPILATION

diff --cc src/coreclr/src/vm/excep.cpp
Simple merge

diff --cc src/coreclr/src/vm/jitinterface.cpp
Simple merge