[flang][hlfir] Lower some character elemental references

Lower character elemental user procedures with constant length, and
bot dynamic and constant length ADJUSTL, ADJUSTR, and MERGE references
(which leaves out MIN/MAX).

Character elemental user procedures with dynamic length are a bit more
involving and since it is an edge-case that is not currently supported,
I will take this on later.

Differential Revision: https://reviews.llvm.org/D141847