[NFC][MLGO] Use LazyCallGraph::Node to track functions.

This avoids the InlineAdvisor carrying the responsibility of deleting
Function objects. We use LazyCallGraph::Node objects instead, which are
stable in memory for the duration of the Module-wide performance of CGSCC
passes started under the same ModuleToPostOrderCGSCCPassAdaptor (which
is the case here)

Differential Revision: https://reviews.llvm.org/D116964