compiler.go: createBuiltin: accept alias for slice.

[dkegel-fastly]

Seems to get 1.18 tests closer to passing.

I don't know llvm, so this was a bit of a guess.

For #2515 / #2718

[aykevl]

There is a better way to fix this:

- switch argTypes[0].(type) {
+ switch argTypes[0].Underlying().(type) {

This strips off the named type and makes the type switch work directly with the underlying type (which should be a *types.Chan or *types.Slice).

[aykevl]

Looks good to me. A test would be good but I couldn't get one to work, the following already works for me:

package main

type Buffer []byte

func main() {
	l := make(Buffer, 3, 5)
	println(len(l), cap(l))
}

func foo(l Buffer) {
	println(len(l), cap(l))
}

But it's small and matches the len builtin, so merging.