When instantiating a UnaryOperator, allow the resulting expression to

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@86087 91177308-0d34-0410-b5e6-96231b3b80d8