Add DwarfUnit::addGlobalType to match DwarfUnit::addGlobalName

(these will shortly become virtual, with a null implementation in
DwarfUnit (since type units don't have accelerator tables in the current
schema) and the current implementation down in DwarfCompileUnit, moving
the actual maps there too)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221082 91177308-0d34-0410-b5e6-96231b3b80d8