[flang] Fix source allocation to explicit length after deferred length object (#87785)

Flang supports source allocation to allocatable or pointers with a non
deferred length that do not match the source length. This documented at:
https://github.com/llvm/llvm-project/blob/9708d0900311503aa4685d6810d8caf0412e15d7/flang/docs/Extensions.md?plain=1#L312

The current lowering code was bugged when such explicit length allocate
object appeared after a deferred length object in the source allocation
list:

Since "lenParams" had been computed when generating allocation of the
deferred length object, the call to genSetDeferredLengthParameters was
not a no-op on when lowering the explicit length allocation, and the
explicit length was overridden with the source length.

The output of the program added in test was:

```
ZZheZZ
ZZhelloZZ
ZZhelloZZ
```

Instead of:

```
ZZheZZ
ZZhelloZZ
ZZhello  ZZ
```

Skip genSetDeferredLengthParameters when the allocate object has non
deferred length.