[memprof] Filter out callstack frames which cannot be symbolized.

This patch filters out callstack frames which can't be symbolized or if
the frames belong to the runtime. Symbolization may not be possible if
debug information is unavailable or if the addresses are from a shared
library. For now we only support optimization of the main binary which
is statically linked to the compiler runtime.

Differential Revision: https://reviews.llvm.org/D120860