Use ProfileSummary:getProfileCount to get ScaledCount for ModuleSummary

Summary: ModuleSummary should use the standard interface of ProfileSummary::getProfileCount.

Reviewers: eraman, tejohnson

Reviewed By: tejohnson

Subscribers: tejohnson, mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D31154

llvm-svn: 298404