Add a better error message when failing to assign one tablegen value to another

Add a better error message when failing to assign one tablegen value to another

This is currently for assigning from one bit init to another.  It can easily be extended to other types.

Test to follow soon in another patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214374 91177308-0d34-0410-b5e6-96231b3b80d8