wire: allow wire.Value to use values with no parent (i.e., struct fields) (google/go-cloud#596)

internal/wire/testdata/ValueIsStruct/want/wire_gen.go

@@ -0,0 +1,17 @@
+// Code generated by Wire. DO NOT EDIT.
+
+//go:generate wire
+//+build !wireinject
+
+package main
+
+// Injectors from wire.go:
+
+func injectFoo() Foo {
+	foo := _wireFooValue
+	return foo
+}
+
+var (
+	_wireFooValue = Foo{X: 42}
+)