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

[netcore] Require use of MonoClass getters in Debug builds (mono/mono#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.

Commit migrated from https://github.com/mono/mono/commit/6abab691ec71c3cb4f72dd6d8f630088c8330357