* mcs/const.cs (Constant.CheckBase): New override that defers the
new-or-override check in case the base type hasn't been populated yet.
(Constant.Define): Ensure the new-or-override check is performed.
* tests/test-341.cs: Update to reflect fix of compiler crash.
* errors/cs0108-12.cs: New test. Ensure that cs0108 handling occurs when
a constant is defined ahead of time due to forward references.
svn path=/trunk/mcs/; revision=39971
