[dev.typeparams] cmd/compile: simplify inlining variadic calls

We already have and use FixVariadicCall to normalize non-dotted calls
to variadic functions elsewhere in the compiler to simplify rewriting
of function calls. This CL updates inl.go to use it too.

A couple tests need to be updated to (correctly) expect diagnostics
about "... argument" instead of a slice literal. This is because
inl.go previously failed to set Implicit on the slice literal node.

Change-Id: I76bd79b95ae1f16e3b26ff7e9e1c468f538fd1f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/323009
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

test/inline_variadic.go

@@ -14,6 +14,6 @@ func head(xs ...string) string { // ERROR "can inline head" "leaking param: xs t
 }
 
 func f() string { // ERROR "can inline f"
-	x := head("hello", "world") // ERROR "inlining call to head" "\[\]string{...} does not escape"
+	x := head("hello", "world") // ERROR "inlining call to head" "\.\.\. argument does not escape"
 	return x
 }