Add more verification on consectutive bitfields otherwise clang will assert.

We need to verify that consecutive bitfields have higher offsets and don't overlap. The issues was found by running a broken version of recent clangs where the bitfield offsets were being emitted incorrectly. To guard against this we now verify and toss out any invalid bitfields and print a message that indicates to file a bug against the compiler.

<rdar://problem/25737621>

llvm-svn: 271343