Fix PR14881 by implementing conversion rules between int and complex int.

Prior to the patch, Clang does not properly promote types when a complex
integer operand is combined with an integer via a binary operator, or when
one is assigned to the other in either order.  This patch detects when
promotion is needed (and permissible) and generates the necessary code.

The test assmes no target has the same size operands for "char" and
"long long," and that no target performs arithmetic on char operands without
extending them to a larger format first.  If there are any targets for
which this is not the case, they should be XFAILed.