Auto merge of #82834 - nikic:mutable-noalias, r=nagisa

Enable mutable noalias for LLVM >= 12

Enable mutable noalias by default on LLVM 12, as previously known miscompiles have been resolved. Now it's time to find the next one ;)

 * The `-Z mutable-noalias` option no longer has an explicit default and accepts `-Z mutable-noalias=yes` and `-Z mutable-noalias=no` to override the LLVM version based default behavior.
 * The decision on whether to apply the noalias attribute is moved into rustc_codegen_llvm. rustc_middle only provides us with the necessary information to make the decision.
 * `noalias` is not emitted for types that are `!Unpin`, as a heuristic for self-referential structures (see #54878 and #63818).

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