[flang] Create a temporary of the correct size when lowering SetLength

This patch creates a temporary of the appropriate length while lowering SetLength.

The corresponding character can be truncated or padded if necessary.

This fix issue with array constructor in argument and also with statement function.

```
  character(7) :: str = "1234567"
  call s(str(1:1))
contains
 subroutine s(a)
  character(*) :: a
  call s2([Character(3)::a])
 end subroutine
 subroutine s2(c)
  character(3) :: c(1)
  print "(4a)", c(1), "end"
 end subroutine
end
```

The example prior the patch prints `123end` instead of `1. end`

Reviewed By: PeteSteinfeld, jeanPerier

Differential Revision: https://reviews.llvm.org/D132464