test(std): Add codegen test for array::from_fn optimization

This commit adds a new test file 'array-from_fn.rs' to the codegen test suite.
The test checks the behavior of std::array::from_fn under different optimization levels:

1. At opt-level=0 (debug build), it verifies that the core::array::Guard
   is present in the generated code.
2. At opt-level=s (size optimization), it ensures that the Guard is
   optimized out.

This test helps ensure that the compiler correctly optimizes array::from_fn
calls in release builds while maintaining safety checks in debug builds.

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