Rename ST_External to ST_Unknown, and slightly change its semantics. It now only indicates that the symbol's type
is unknown, not that the symbol is undefined. (For that, use ST_Undefined).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151696 91177308-0d34-0410-b5e6-96231b3b80d8