[Sample Profile Loader] Fix potential invalidated reference (#73181)

There is a potential issue in ProfiledCallGraph where pointers to
ProfiledCallGraphNode are used to construct edges, while
ProfiledCallGraphNode instances are being added to a hash map
ProfiledFunctions simultaneously. If rehash happens, those pointers are
invalidated, resulting in undefined behavior/crash. Previously (before
md5phase2) ProfiledFunctions is a llvm::StringMap, which also have the
same issue theoretically when rehashing but was not observed. This patch
fixes this potential issue by using a backing buffer for
ProrfiledCallGraphNode that does not relocate.