Handle external interface in convert_call_args

[Pranavchiku]

Fixes #1977. Fixes #2748. Fixes #2427

[Pranavchiku]

Generated LLVM looks good to me.

[Pranavchiku]

This case do not work yet:

subroutine dqc25f()
   external f
   call dqk15w(f)
end

subroutine dqk15w(f)
   interface
      function f(x) result (r)
         real r, x
      end function
   end interface
   real r
   r = f(2.0)
   print *, r
end subroutine

program main
   call dqc25f()
end program

Because, interface of f is updated in subroutine dqk15w , right now we only append to changed_external_function_symbol if it is external interface. It is easy fix.

[certik]

Here is it calling f.1, meaning that it won't be able to find the definition of this at link time, correct?

[certik]

Can you try to compile and link an example? I think it won't work, due to a missing f.1 symbol.

[Pranavchiku]

This case works correctly: #2748 (comment)

Here it has concrete definition of f

[certik]

How exactly does the f.1 symbol get resolved? We need to understand that.

[certik]

@Pranavchiku to move forward:

• can you add an integration test that fails in master but now works? Then we can merge this.
• As a separate investigation, we need to understand what is happening with the f.1.

[Pranavchiku]

I added the test, this PR can have a review.

[certik]

I think this looks good now, thanks!