[dev.typealias] cmd/gofmt: added test cases for alias type declarations

For #18130.

Change-Id: I95e84130df40db5241e0cc25c36873c3281199ff
Reviewed-on: https://go-review.googlesource.com/34987Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/gofmt/testdata/typealias.golden

+package q
+
+import "p"
+
+type _ = int
+type a = struct{ x int }
+type b = p.B
+
+type (
+	_  = chan<- int
+	aa = interface{}
+	bb = p.BB
+)
+
+// TODO(gri) We may want to put the '=' into a separate column if
+// we have mixed (regular and alias) type declarations in a group.
+type (
+	_   chan<- int
+	_   = chan<- int
+	aa0 interface{}
+	aaa = interface{}
+	bb0 p.BB
+	bbb = p.BB
+)

src/cmd/gofmt/testdata/typealias.input

+package q
+
+import "p"
+
+type _ = int
+type a = struct{ x int }
+type b = p.B
+
+type (
+	_ = chan<- int
+	aa = interface{}
+	bb = p.BB
+)
+
+// TODO(gri) We may want to put the '=' into a separate column if
+// we have mixed (regular and alias) type declarations in a group.
+type (
+	_ chan<- int
+	_ = chan<- int
+	aa0 interface{}
+	aaa = interface{}
+	bb0 p.BB
+	bbb = p.BB
+)