[IR] Make Module::setProfileSummary to replace an existing ProfileSummary flag.

Summary:
Module::setProfileSummary currently calls addModuelFlag. This prevents from
updating the ProfileSummary metadata in the module and results in a second
ProfileSummary added instead of replacing an existing one. I don't think this is
the expected behavior. It prevents updating the ProfileSummary and it does not
make sense to have more than one. To address this, add Module::setModuleFlag and
use it from setProfileSummary.

Reviewers: davidxl

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79902