Use 'unsigned' for enum bitfields

Fixes this clang warning on Windows:

warning: implicit truncation from 'clang::LangOptions::FPContractModeKind' to bit-field changes value from 2 to -2 [-Wbitfield-constant-conversion]
    fp_contract = LangOptions::FPC_Fast;
                ^ ~~~~~~~~~~~~~~~~~~~~~
llvm-svn: 299045