[flang] Unsplit COMPLEX operations

COMPLEX negation, addition, subtraction, conversions of kind, and
equality/inequality were represented as component-wise REAL
operations.  It turns out to be easier for lowering if we
do not split and recombine these COMPLEX operations, and it
avoids a potential problem with COMPLEX valued function calls
in these contexts.  So add this suite of operations to the
typed expression representation in place of the component-wise
transformations, and support them in folding.

Differential revision: https://reviews.llvm.org/D91443