[LTO] Enable module summary emission by default for regular LTO

Summary:
With D33921, we gained the ability to have module summaries in regular
LTO modules without triggering ThinLTO compilation. Module summaries in
regular LTO allow garbage collection (dead stripping) before LTO
compilation and thus open up additional optimization opportunities.

This patch enables summary emission in regular LTO for all targets
except ld64-based ones (which use the legacy LTO API).

Reviewers: pcc, tejohnson, mehdi_amini

Subscribers: inglorion, eraman, cfe-commits

Differential Revision: https://reviews.llvm.org/D34156

llvm-svn: 335284