internal/analysisinternal: TypeExpr: don't forget Variadic

The syntax generated for type func(...T) was func([]T),
causing extract operations to misbehave.

Fixes golang/go#63287

Change-Id: I37978a13b2b51f0802ea694b7711a7d6444384fb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/555458
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

gopls/internal/test/marker/testdata/codeaction/extract-variadic-63287.txt

@@ -0,0 +1,28 @@
+This test exercises extract on a variadic function.
+It is a regression test for bug #63287 in which
+the final paramater's "..." would go missing.
+
+-- go.mod --
+module testdata
+go 1.18
+
+-- a/a.go --
+package a
+
+//@codeactionedit(block, "refactor.extract", out, "Extract function")
+
+func _() {
+	var logf func(string, ...any)
+	{ println(logf) } //@loc(block, re`{.*}`)
+}
+
+-- @out/a/a.go --
+@@ -7 +7 @@
+-	{ println(logf) } //@loc(block, re`{.*}`)
++	{ newFunction(logf) } //@loc(block, re`{.*}`)
+@@ -10 +10,4 @@
++func newFunction(logf func( string,  ...any)) {
++	println(logf)
++}
++
+-- end --

internal/analysisinternal/analysis.go

@@ -151,6 +151,10 @@ func TypeExpr(f *ast.File, pkg *types.Package, typ types.Type) ast.Expr {
 				},
 			})
 		}
+		if t.Variadic() {
+			last := params[len(params)-1]
+			last.Type = &ast.Ellipsis{Elt: last.Type.(*ast.ArrayType).Elt}
+		}
 		var returns []*ast.Field
 		for i := 0; i < t.Results().Len(); i++ {
 			r := TypeExpr(f, pkg, t.Results().At(i).Type())