Fix a latent bug exposed by recent static member debug info changes.

We weren't encoding boolean constants correctly due to modeling boolean as a
signed type & then sign extending an i1 up to a byte & getting 255.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172926 91177308-0d34-0410-b5e6-96231b3b80d8