go.tools/go/types: re-disable float_lit2.go std test
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://codereview.appspot.com/96540045

chenjianlong/go.tools,karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,chenjianlong/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools