Add a warning when assigning enums to bitfields without an explicit unsigned underlying type. This is to prevent problems with MSVC compatibility, since the Microsoft ABI defaults to storing enums with a signed type, causing inconsistencies with saving to/reading from bitfields.
Also disabled the warning in the dr0xx.cpp test which throws the error, and added a test for the warning.
The warning can be disabled with -Wno-signed-enum-bitfield.