Remove static mut refs from drop tests

[NotPppp1116]

Replace several static mut counters in drop and union UI tests with atomic counters.

These tests only need simple observable drop counts/order within a single test run. Using AtomicUsize/AtomicU8 with relaxed ordering preserves that behavior while removing stale #![allow(static_mut_refs)] allowances and the associated unsafe counter accesses.

Validation:

./x test tests/ui/drop/drop-count-assertion-16151.rs tests/ui/drop/same-alloca-reassigned-match-binding-16151.rs tests/ui/drop/destructor-run-for-expression-4734.rs tests/ui/drop/destructor-run-for-let-ignore-6892.rs tests/ui/drop/conditional-drop-10734.rs tests/ui/drop/repeat-drop.rs tests/ui/union/union-drop.rs tests/ui/union/union-drop-assign.rs

Result: 8 passed, 0 failed.

[asquared31415]

Not all targets support AtomicUsize::fetch_add, and I'm not sure if that is a concern for testing. I believe some (especially obscure T3) targets cross-compile the tests to their target, and the use of AtomicUsize::fetch_add could cause them to no longer be able to test. If the compiler team wants these tests to continue working for all targets, they may need to be disabled on non-atomic targets, or another approach should be used.

Edit: oh i just saw the review in #159144, the same concerns apply here.

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

---- [ui] tests/ui/drop/conditional-drop-10734.rs stdout ----

error: test compilation failed although it shouldn't!
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/drop/conditional-drop-10734.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "-O" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/drop/conditional-drop-10734/a" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error[E0530]: let bindings cannot shadow statics
##[error]  --> /checkout/tests/ui/drop/conditional-drop-10734.rs:10:13
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