Add Windows (clang-cl) support for continuous fuzzing

[posidron]

Summary

Enable continuous fuzzing (--fuzz=) on Windows with clang-cl.

Problem

FuzzTest's continuous fuzzing mode fails on Windows with:

Despite clang-cl correctly applying -fsanitize-coverage=inline-8bit-counters and the __sanitizer_cov_8bit_counters_init callback firing, the coverage data is never used because FUZZTEST_COVERAGE_IS_AVAILABLE excludes _WIN32.

CorpusCoverage::Update() compiles as a stub returning false on Windows, so the fuzzer never considers any input as providing new coverage.

Changes

1. Add defined(_WIN32) to the FUZZTEST_COVERAGE_IS_AVAILABLE guard - clang-cl supports the same sanitizer coverage instrumentation as clang on Linux/macOS.

2. Use _aligned_malloc/_aligned_free on Windows — the MSVC CRT does not provide std::aligned_alloc. Memory allocated with _aligned_malloc must be freed with _aligned_free (not std::free).

[google-cla]

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[posidron]

@fniksic How can we merge this? I noticed you are red for the CLA.

[fniksic]

@fniksic How can we merge this? I noticed you are red for the CLA.

The CLA check fails because after you incorporated my suggested fix, GitHub added me as a co-author using my private email instead of my Google email. Since I'm a Google employee, I can't really sign the CLA in the same way, so I can't fix it that way.

Can you remove me as a co-author? See these instructions: https://github.com/orgs/community/discussions/76660#discussioncomment-7649637

[posidron]

@fniksic Thank you. Ready for merge. :)