[DebugInfo][flang]Added support for representing Fortran assumed length strings

This patch adds support for representing Fortran `character(n)`.

Primarily patch is based out of D54114 with appropriate modifications.

Test case IR is generated using our downstream classic-flang. We're in process
of upstreaming flang PR's but classic-flang has dependencies on llvm, so
this has to get in first.

Patch includes functional test case for both IR and corresponding
dwarf, furthermore it has been manually tested as well using GDB.

Source snippet:
```
 program assumedLength
   call sub('Hello')
   call sub('Goodbye')
   contains
   subroutine sub(string)
           implicit none
           character(len=*), intent(in) :: string
           print *, string
   end subroutine sub
 end program assumedLength
```

GDB:
```
(gdb) ptype string
type = character (5)
(gdb) p string
$1 = 'Hello'
```

Reviewed By: aprantl, schweitz

Differential Revision: https://reviews.llvm.org/D86305