fuzz: fix filesystem bug, use std::__fs::filesystem (#7507)
fuzz: fix filesystem bug, use std::__fs::filesystem (#7507)

Since OSS-Fuzz minijail prohibits mkdir calls, we use std::__fs::filesystem namespace which implements filesystem.

The OSS-Fuzz image was taking the mkdir branch, since neither filesystem nor experimental/filesystem was found. However, we are able to use std::__fs::filesystem branch in the OSS-Fuzz image and the std::experimental::filesystem branch locally.

Risk Level: Low
Testing: Add RELEASE_ASSERT(1==0, "") under the #if defined(_LIBCPP_VERSION) && TEST_STD_VER < 17 branch to verify that branch is taken in oss-fuzz docker. run_fuzzer with oss-fuzz docker is successful.
Fixes OSS-Fuzz issue:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=15414

Signed-off-by: Asra Ali <da4828c05ee1cc5a64445440d12151dedd1ac4a8@google.com>

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