DwarfUnit: Make explicit a limitation/bug in enumeration constant emission.

Filed as PR19712, LLVM fails to detect the right type of an enum
constant when a frontend does not provide an underlying type for the
enumeration type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208502 91177308-0d34-0410-b5e6-96231b3b80d8