[flang] Fix COMPLEX/REAL `/` and `**` operand promotion

The real operand was always converted to the complex operand type.
The highest precison should be used instead. This fix converts the
real to a complex of the same kind before applying the promotion
rules for complex operands.
Reference to Fortran 2018 standard 10.9.1.3 that rules this added
in comments.

Original-commit: flang-compiler/f18@0d6b9c33aae59d80ff1b77f3561a6acb1db98645
Reviewed-on: https://github.com/flang-compiler/f18/pull/858