[SampleProfile] Potential use after move in...

[SampleProfile] Potential use after move in SampleProfileLoader::promoteMergeNotInlinedContextSamples

SampleProfileLoader::promoteMergeNotInlinedContextSample adds
certain uninlined functions to the sample profile map (unordered_map, which is
previously read from a profile file). This action may cause the map to
be rehashed, invalidating all pointers to FunctionSamples used by many
members of SampleProfileLoader, while the existing code did nothing to
guard against that. This bug is theoretical since adding a few
new functions to a large profile usually won't trigger a rehash, or even
if there's a rehash std::unordered_map tries its best to expand its
capacity in-place.

This bug will trigger if the container type of sample profile map is
changed to llvm::DenseMap or other implementation, such as in D147740,
for SampleProfReader's performance reason.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D157061