[flang/flang-rt] Implement show_descriptor intrinsic, a non-standard extension. (#170389)
This is a reapply the original patch (#169137) with the flang-rt unit
test changes limiting it to linux platform only.
Additionally accommodated style changes from Peter Klausler (#170227)
show_descriptor intrinsic prints details of a descriptor (extended
Fortran pointer).
It accepts a descriptor for any type and rank, including scalars.
Requires use of flang_debug module.
Example:
```
program test
use flang_debug
implicit none
integer :: a(4) = (/ 1,3,5,7 /)
call show_descriptor(a(1:3))
end program test
```
and its output:
```
Descriptor @ 0x7ffe01ec6a98:
base_addr 0x563b7035103c
elem_len 4
version 20240719
rank 1
type 9 "INTEGER(kind=4)"
attribute 0
extra 0
addendum 0
alloc_idx 0
```
GitOrigin-RevId: 4b2714f12f592ef3237056759dbe735d43fbeec1
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