Enables `If` operator lowering in cond_v2 when XLA is disabled. Lowering allows cond_v2 to avoid some of the limitations of Functions, allowing users to specify devices & colocation inside of cond_v2 branches, and enabling non-strict evaluation & partial pruning of branches. This brings cond_v2 closer to feature parity with tf.cond.

However, we do not lower `If` in the XLA context because it is easier for XLA to apply its own optimizations when dealing with un-lowered `If` operators than with lowered switch/merge control flow.

Also adds a toggleable flag in for InlineFunctionBody in function.cc that prevents the function caller device from overriding the devices of function body nodes. This is necessary for cond_v2 branches to support explicitly-specified devices.

Adds several tests to make sure that:
- lowering is usually enabled
- lowering is disabled for XLA
- node colocation inside of cond_v2 branches works
- explicit device placement inside of cond_v2 branches works

PiperOrigin-RevId: 201049850

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