[llvm] [llvm][TypeSize] Fix addition/subtraction in TypeSize. (PR #72979)

[Sander de Smalen via llvm-commits]

sdesmalen-arm wrote:

@gchatelet I've split out the new feature (adding any value to zero-initialized TypeSize) into a separate PR #72994.
@paulwalker-arm I'm happy to rename the name of TypeSize::Scalable -> TypeSize::getScalable (or something) if there is consensus on the name, but I'd prefer do that in a separate NFC patch.

https://github.com/llvm/llvm-project/pull/72979