Revert "Declare GlobalValue::LinkageTypes based on unsigned."

This reverts commit r267335.  The build has been broken for hours
because of it:

  http://lab.llvm.org:8080/green/job/clang-stage1-cmake-RA-incremental_build/23352/

The correct fix is avoid using any enum in a bitfield.

llvm-svn: 267341