[flang] Upstream lowering of real control loops

Upstream the code for handling loops with real control variables from
the fir-dev branch at
https://github.com/flang-compiler/f18-llvm-project/tree/fir-dev/



Also add a test.

Loops with real-valued control variables are always lowered to
unstructured loops. The real-valued control variables are handled the
same as integer ones, the only difference is that they need to use
floating point instructions instead of the integer equivalents.

Co-authored-by: V Donaldson <vdonaldson@nvidia.com>