Fix missing const for inherent pointer replace methods

[Sky9x]

ptr::replace (the free fn) is already const stable. However, there are inherent convenience methods on *mut T and NonNull<T>, allowing you to write eg. unsafe { foo.replace(bar) } where foo is *mut T or NonNull<T>.

It seems const was never added to the inherent method (likely oversight), so this PR adds it.
I don't believe this needs another¹ FCP as the inherent methods are already stable and ptr::replace is already const stable, so this adds no new API.

Original tracking issue: #83164
ptr::replace constified in #83091
ptr::replace const stabilized in #130954

Footnotes

1. const_replace FCP completed: https://github.com/rust-lang/rust/issues/83164#issuecomment-2385670050 ↩

[rustbot]

r? @jhpratt

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[Sky9x]

@rustbot label +T-libs-api

[jhpratt]

I agree that this seems like an oversight and doesn't need another FCP, but I'll defer to @rust-lang/libs-api as it is ultimately their decision. r=me if they're fine with it.

[dtolnay]

@rfcbot fcp merge

[rfcbot]

Team member @dtolnay has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

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