In https://reviews.llvm.org/D136765 / https://reviews.llvm.org/D144155,
the asan annotations for `std::vector` were modified to unpoison freed
backing memory on destruction, instead of leaving it poisoned. However,
calling `__clear()` instead of `clear()` skips informing the asan runtime
of this decrease in the accessible container size, which breaks the
invariant that the value of `old_mid` should match the value of `new_mid`
from the previous call to `__sanitizer_annotate_contiguous_container`, which
can trip the sanity checks for the partial poison between [d1, d2) and the
container redzone between [d2, c), if enabled. To fix this, ensure that
`clear()` is called instead, as is already done by `__vdeallocate()`.
Also remove `__clear()`, since it is no longer called.
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