Consistently use int64_t for shape-related values in shaped types

    We want to support 64-bit shapes (even when the compiler is on a 32-bit architecture). Using int64_t consistently allows us to sidestep the bugginess of unsigned arithmetic.

    Still unsigned: kind, memory space, and bit width. The first two are basically enums. We could have a discussion about the last one, but it's basically just a very large enum as well and we're not doing any math on it, I think.

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PiperOrigin-RevId: 250985791