go/ir: actually build blank functions

In d074ce0 we stopped dropping objects
with blank names on the floor. However, we forgot to actually build
functions with blank names, therefore exposing malformed Function
objects to users of go/ir.

Closes: gh-1572

go/ir/builder.go

@@ -3303,9 +3303,6 @@ func (b *builder) buildYieldFunc(fn *Function) {
 // by decl in package pkg.
 func (b *builder) buildFuncDecl(pkg *Package, decl *ast.FuncDecl) {
 	id := decl.Name
-	if isBlankIdent(id) {
-		return // discard
-	}
 	fn := pkg.values[pkg.info.Defs[id]].(*Function)
 	if decl.Recv == nil && id.Name == "init" {
 		var v Call