DwarfUnit: Pick a winner between isTypeSigned and isUnsignedDIType.

And the winner by a nose is isUnsignedDIType, for no particular reason.

These two functions were just complements of each other and used in very
related code, so refactor callers to just use one of them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208500 91177308-0d34-0410-b5e6-96231b3b80d8