[BOLT] Free memory for CFG after emission

Summary:
Once we emit function code, we no longer need CFG for next phases
that use basic blocks for address-translation and symbol update
purposes. We free memory used by CFG and instructions. The freed
memory gets reused by later phases resulting in overall memory usage
reduction.

We can probably improve memory consumption even further by replacing
BinaryBasicBlocks with more compact data structures.

(cherry picked from FBD18408954)