PointerIntPair: Avoid an (academic) case of undefined behavior in the DenseMapInfo specialization.

If we use a pair with an enum type this could create values outside
of the enum range. Avoid it by creating the bit pattern directly.
While there turn a dynamic assert into a static one. No functionality
change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204010 91177308-0d34-0410-b5e6-96231b3b80d8