[netcore] Require use of MonoClass getters in Debug builds (#18267)

1. Update `build.sh` to use `--enable-checked-build=private_types` which forces the use of getter methods to access `MonoClass` fields.
2. Fix up some places that were accessing MonoClass directly.
3. Get rid of a spurious comma operator usage from an earlier refactoring:
   ```c
      z = a, b, c;
   ```
   is the same as
   ```c
      z = a; b; c;
   ```
   which is the same as just `z = a;` if `b` and `c` have no side-effects.

0 files changed, 0 insertions, 0 deletions