gc: tweak and enable escape analysis

gc: tweak and enable escape analysis

-s now means *disable* escape analysis.

Fix escape leaks for struct/slice/map literals.
Add ... tracking.
Rewrite new(T) and slice literal into stack allocation when safe.

Add annotations to reflect.
Reflect is too chummy with the compiler,
so changes like these affect it more than they should.

R=lvd, dave, gustavo
CC=golang-dev
https://golang.org/cl/4954043