[llvm-profgen] Fix a dangling vector reference in CS line number based generator

It seems we missed one spot to persist `SampleContextFrameVector` into the global table (CSProfileGenerator::populateFunctionBoundarySamples:340) which causes a crash.

This change tried to fix it in a centralized way i. e. where we generate the `FunctionSamples`.

Reviewed By: hoy, wenlei

Differential Revision: https://reviews.llvm.org/D110275