[memprof] Fix use-after-free in peekBuildIds.

To check the uniqueness of buildids, we held on to a StringRef of the build id string pushed into the vector. If the number of build ids were large enough to trigger a realloc in the vector then these references where invalidated resulting in a use-after free. This was exposed in downstream usage.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D155110