[ProfileSummary] Add the PartialProfileRatio field in ProfileSummary metadata.

Summary:
PartialProfileRatio approximately represents the ratio of the number of profile
counters of the program being built to the number of profile counters in the
partial sample profile. It is used to scale the working set size under the
partial sample profile to reflect the size of the program being built and to
improve the working set size heuristics.

This is a split from D79831.

Reviewers: davidxl

Subscribers: eraman, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79951