Rollup merge of #99011 - oli-obk:UnsoundCell, r=eddyb

`UnsafeCell` blocks niches inside its nested type from being available outside

fixes #87341

This implements the plan by `@eddyb` in https://github.com/rust-lang/rust/issues/87341#issuecomment-886083646

Somewhat related PR (not strictly necessary, but that cleanup made this PR simpler): #94527

compiler/rustc_span/src/symbol.rs

@@ -980,7 +980,6 @@ symbols! {
         no_link,
         no_main,
         no_mangle,
-        no_niche,
         no_sanitize,
         no_stack_check,
         no_start,
@@ -1153,7 +1152,6 @@ symbols! {
         repr128,
         repr_align,
         repr_align_enum,
-        repr_no_niche,
         repr_packed,
         repr_simd,
         repr_transparent,