Support conversion to interface type in function calls

[dlsniper]

1. What version of Delve are you using (dlv version)?
   72fae3c
2. What version of Go are you using? (go version)?
   Go 1.13 (ac8dbe7747971007d58eb39e2e7e615cf9f04493)
3. What operating system and processor architecture are you using?
   Win 10
4. What did you do?
   Run the following application with a breakpoint on line 8

package main

import "fmt"

func main() {
	colors := map[string]string{"key":"value"}
	color, ok := colors["ke"]
	if !ok {
		// handle missing map key case here
	}
	fmt.Println(color)
}

Invoke the following delve command:

call fmt.Println(color)

or

call fmt.Println(ok)

5. What did you expect to see?
   I would expect the debugger show the result of the function call and print it on the output
6. What did you see instead?

Command failed: can not convert value of type string to []interface {}

or

Command failed: can not convert value of type boolean to []interface {}

[chainhelen]

I found the same issue yesterday. In fact, not support variadic first here, then interface type.

[icholy]

I wonder if there's a way to do this without injecting calls to runtime·convT2E in the target.

[zuozhiw]

Bump on this issue. I find it really inconvenient to use. In normal code, this would work perfectly fine, but not in a debugger. Often, the variable is a concrete type rather than an interface type, and we need to pass it to a more general function, like printing. It would be great to have an update or resolution for this.