To support variadic function call evaluation

1. What version of Delve are you using (dlv version)?

Version: 1.5.0
Build: 46d344431a02d201821e665b486caeecff47b558

1. What version of Go are you using? (go version)?

go1.15

1. What operating system and processor architecture are you using?

windows, amd64

1. What did you do?

Tried to evaluate a variadic function call, e.g. filepath.Join("a", "b", "c")

1. What did you expect to see?

result of function call

1. What did you see instead?

Got an error: too many arguments