Run frontend driver regressions in isolated LLVM backend CI jobs

[LunaStev]

Gap

The frontend driver tests introduced in #486 select an enabled LLVM target, so they can run in isolated backend builds. However, .github/workflows/rust.yml currently runs the isolated WebAssembly and LoongArch jobs with only --test codegen_regressions and a name filter (webassembly_ or loongarch64_). Those steps never execute tests/frontend_regressions.rs. The default workspace and full core64 lanes cover these tests, but cannot catch accidental reliance on another enabled backend in the isolated configurations.

This is an additional CI coverage task after #486, not a request to repair a currently failing frontend test.

Small contribution

Add a separate frontend driver step to each existing isolated WebAssembly and LoongArch job:

cargo test --locked --no-default-features --features llvm-target-wasm --test frontend_regressions --jobs 2
cargo test --locked --no-default-features --features llvm-target-loongarch --test frontend_regressions --jobs 2

Run each command in its corresponding existing job after toolchain/stdlib setup. Do not pass the codegen test-name filter to these commands: it would select zero frontend tests. No new runner, backend implementation, or target matrix is needed.

On #486's local head 6fb6ed7, WebAssembly frontend tests passed 4/4 and the LoongArch frontend suite passed 3/3 during validation.

Acceptance

• Both isolated jobs execute the frontend test binary and report a nonzero test count.
• Existing codegen/ABI smoke steps remain intact.
• The WebAssembly suite exercises wasm32 versus wasm64 pointer-sized integer behavior.
• Commands retain --locked and --jobs 2.

Start in .github/workflows/rust.yml and tests/frontend_regressions.rs. This is suitable for a first contribution because the tests and required toolchains already exist.