go.tools/types: skip "imported but not used" check if IgnoreFuncBodies.
go.tools/types: skip "imported but not used" check if IgnoreFuncBodies.

Obviously in that mode, we can't correctly diagnose such
errors, so we shouldn't attempt it (and emit false positives).

R=gri
CC=golang-dev
https://codereview.appspot.com/41080043

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