[flang] Lower procedure ref to user defined elemental procedures (part 1)

[flang] Lower procedure ref to user defined elemental procedures (part 1)

Lower procedure ref to user defined elemental procedure when:
- there are no arguments that may be dynamically optional
- for functions, the result has no length parameters
- the reference can be unordered
- there are not character by value arguments

This uses the recently added hlfir.elemental operation and tools.
The "core" of the argument preparation is shared between elemental
and non elemental calls (genUserCalls is code moved without any
functional changes)

Differential Revision: https://reviews.llvm.org/D140118