Add an OperandNamespace field to Target.td's Operand.

Add an OperandNamespace field to Target.td's Operand.

For targets to add their own operand types as needed, as advertised in
Operand's comment, they need to be able to specify an alternate namespace
for OperandType names too. This matches the RegisterOperand class.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256299 91177308-0d34-0410-b5e6-96231b3b80d8