[BOLT] Detect and handle __builtin_unreachable().

Summary:
Calls to __builtin_unreachable() can result in a inconsistent CFG.
It was possible for basic block to end with a conditional branche
and have a single successor. Or there could exist non-terminated
basic block without successors.

We also often treated conditional jumps with destination past the end
of a function as conditional tail calls. This can be prevented
reliably at least when the byte past the end of the function does
not belong to the next function.

This diff includes several changes:
  * At disassembly stage jumps past the end of a function are converted
    into 'nops'. This is done only for cases when we can guarantee that
    the jump is not a tail call. Conversion to nop is required since the
    instruction could be referenced either by exception handling
    tables and/or debug info. Nops are later removed.
  * In CFG insert 'ret' into non-terminated basic blocks without
    successors (this almost never happens).
  * Conditional jumps at the end of the function are removed from
    CFG. The block will still have a single successor.
  * Cases where a destination of a jump instruction is the start
    of the next function, are still conservatively handled as
    (conditional) tail calls.

(cherry picked from FBD4655046)