Rollup merge of #88722 - WaffleLapkin:unsafe_cell_const_get_mut, r=dtolnay
Rollup merge of #88722 - WaffleLapkin:unsafe_cell_const_get_mut, r=dtolnay

Make `UnsafeCell::get_mut` const

aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust