go.tools/go/types: re-disable float_lit2.go std test

go/types doesn't correctly round the largest possible
float32 literal values and fails. Instead of relying
on Rat.Float64 and float32 conversion, we need a
Rat.Float32 implementation with correct rounding.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/96540045

go/types/stdlib_test.go

@@ -119,6 +119,7 @@ func TestStdTest(t *testing.T) {
 	testTestDir(t, filepath.Join(runtime.GOROOT(), "test"),
 		"cmplxdivide.go", // also needs file cmplxdivide1.go - ignore
 		"sigchld.go",     // don't work on Windows; testTestDir should consult build tags
+		"float_lit2.go",  // TODO(gri) float32 constant conversion requires (missing) Rat.Float32
 	)
 }