DebugInfo: ensure negative enumerators are emitted as such

DebugInfo: ensure negative enumerators are emitted as such

There's still a problem here - since we're not appropriately using the
signedness/range of the enum to chooset the encoding and emission of
enumerators, but GCC has some bugs around this too so I assume that's
not /such/ a high priority though I may get to it soon out of
completeness.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184695 91177308-0d34-0410-b5e6-96231b3b80d8