Make applying const repeatedly to the same type have less additional effect.

[jonmeow]

This is to avoid edge cases where there are multiple ConstType instructions, which code may not handle appropriately. I was thinking about this for #6279

[danakj]

LGTM

[zygoloid]

I don't think we should do this: if code doesn't handle nested ConstTypes incorrectly, it should be fixed.

Note that the constant value of the type already flattens multiple consts into a single const; this is only dropping the const from the "as-written" form of the type, making the (valid) SemIR not match the written form of the source.