Move the bitcode error enum to the include directory.

This will let users in other libraries know which error occurred. In particular,
it will be possible to check if the parsing failed or if the file is not
bitcode.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214209 91177308-0d34-0410-b5e6-96231b3b80d8