Make this operator bool() explicit to match the standard library.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210073 91177308-0d34-0410-b5e6-96231b3b80d8
Remove the last unspecified_bool_type from llvm.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210076 91177308-0d34-0410-b5e6-96231b3b80d8
Use an enum class.

Might also fix the windows build.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210077 91177308-0d34-0410-b5e6-96231b3b80d8
Use an enum class.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210078 91177308-0d34-0410-b5e6-96231b3b80d8