[flang] Fix generic resolution with actual/dummy procedure incompatib… (#120105)
…ility
We generally allow any legal procedure pointer target as an actual
argument for association with a dummy procedure, since many actual
procedures are underspecified EXTERNALs. But for proper generic
resolution, it is necessary to disallow incompatible functions with
explicit result types.
Fixes https://github.com/llvm/llvm-project/issues/119151.