[CSSPGO] Do not recount callee samples when computing profile summary for nested CS profile.

When generating nested CS profile with all calling contexts of a function duplicated into a base profile under `--generate-merged-base-profiles`, do not recount callee samples when computing profile summary. This fixes the profile summary mismatch between flat cs profile and nested cs profile, for both extbinary and text format.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D119494