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
(cherry picked from commit d0a0c65)

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