Which character is best for killing Bastion from behind Reinhardt?

Bastion fires 35 bullets per second which deal 15 HP damage per each in a close combat, which translates into 525 damage per second. This means Bastion alone will melt the shield in approx. 3.8 seconds at an ideal situation. (My information may not be accurate, however. If that is the case, then just modify the numbers written here.)

However, the real situation is not that ideal. On our plus side, Bastion has damage dropoff which degrades their absurdly high damage output at a range, but the bad news is that Bastion probably has friends around them. I'll assume one standard hero such as McCree or Soldier: 76 would be around them, who has less damage output than Bastion but a significant threat against that gargantuan shield, anyway.

Thus, assuming that Reinhardt's shield lowers after three seconds, these are the heroes who satisfy the DPS bar:

• Pharah. Her rocket deals 120 damage at a direct hit and fires slightly faster than 1 round per second. Passes the damage check by taking less than 2 seconds (from the first rocket fire) for destroying Bastion.
• Bastion themself. Even without math, it is obvious that taking down an enemy Bastion would take less time than breaking Reinhardt's shield. Do note, however, that the offending Bastion might require a delay (which is one second) before stapling themself into the Sentry mode. They could consider installing themself onto a moving object beforehand (e. g. the Payload), or perform the mode switch while falling from a ledge to minimize the effective delay.
• Junkrat, if he can directly hit the target. Same damage as Pharah, with slightly faster fire rate. Note that, however, using him exclusively for this scenario is not an optimal strategy because of his trajectory.
• Hanzo. Each fully charged arrow deals 125 damage, and takes slightly more than 1 second to fully charge and fire. Thus, surprisingly, Hanzo can also pass the damage check when the enemy counterattack is thin aside from Bastion's.

Bonus point for Mercy amping anyone mentioned here, or Zenyatta attaching an Orb of Discord.

Special cases worth a remark:

• Roadhog actually has no means of getting through the DPS bar via shots. However, a test from the training room revealed that he can Chain Hook an enemy as far away as 20 metres, which means he can pull back Bastion safely if they are close enough. Although Hooking alone has minimal impact on Bastion's health, it forces them out of Configuration: Sentry, and pulls them into the close quarter where Reinhardt and Roadhog can shred them with maximum efficiency. Worth noting if your team is close enough.

Pharah can kill a Bastion with three direct rockets at any range (doesn't even need the Reinhardt if she peeks around a corner!), Widowmaker can do the same with three fully charged shots, and Zenyatta can kill a Bastion in 5 normal shots if he uses his Orb of Discord beforehand.

These are probably the three fastest kills, and they work at any range thanks to their lack of damage fall off, and their lack of spread and projectile drop.

Whenever I run into a Bastion + Reinhardt setup, I switch to Symmetra. (And the situation you mention is a subset of this)

There are a few good reasons for this:

• Symmtra's Alt-Fire travels through enemy barriers like Reinhardt's shield, and can deal 125 damage when fully charged (2 seconds). This will kill Bastion in 3 shots. (Assuming no additional healing)
• In addition to the previous point, the Alt-Fire will travel through players as well. It's fully possible to hit Reinhardt's shield, Reinhardt himself, and Bastion if they're all lined up.
• The Alt-Fire is normally hard to hit with because it travels so slowly, but if Bastion is in Turret mode, he can't dodge. And if he leaves Turret mode to avoid your projectiles, you've succeeded at your objective.
• The Alt-Fire doesn't require LOS to charge up; you should line up your shot, duck into cover, then repeatedly charge, duck into sight, fire, and then return to cover while starting to charge up another shot. Repeat liberally.
• Symmetra has shields, so even if Bastion damages her while peeking out, she can simple wait another few seconds to recover from all her damage.

If you're able to ignore the barrier, the Bastion + Reinhardt combo loses a lot of its potency, since both heroes are relying heavily on the barrier to survive -- their entire gameplan is to have Bastion kill your team before your team breaks the barrier. As soon as one or both of them crumble, you've succeeded.

Against a lone Bastion, this strategy is just as good -- without a barrier or healer to back him up, the slow-moving photon balls can often force a Bastion to move -- which resolves the problem all the same.