DwarfUnit: Make explicit a limitation/bug in enumeration constant emission.
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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@208502 91177308-0d34-0410-b5e6-96231b3b80d8

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