Remove a redundant union member.
Remove a redundant union member.

This removes DefinedCOFF from SymbolUnion because DefinedCOFF is
not a leaf class. Pointed out by pcc.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@317013 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld