Add warning when assigning enums to bitfields without an explicit unsigned underlying type

Summary:
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.

Patch by Sasha Bermeister!

Reviewers: rnk, aaron.ballman

Subscribers: mehdi_amini, aaron.ballman, cfe-commits, thakis, dcheng

Differential Revision: https://reviews.llvm.org/D24289

llvm-svn: 287177