When instantiating a UnaryOperator, allow the resulting expression to

still be dependent or invoke an overloaded operator. Previously, we
only supported builtin operators.

BinaryOperator/CompoundAssignOperator didn't have this issue because
we always built a CXXOperatorCallExpr node, even when name lookup
didn't find any functions to save until instantiation time. Now, that
code builds a BinaryOperator or CompoundAssignOperator rather than a
CXXOperatorCallExpr, to save some space.

llvm-svn: 86087