Changes

[clang][driver][NFC] Use StringRef instead of std::string

[utils] The func_dict for a prefix may just be empty

Follow up from D92965 - since we try to find failed prefixes
after each RUN line, it's possible the whole list of functions for a
prefix be non-existent, which is fine - this happens when none of the
RUN lines seen so far used the prefix.

[libc++] Fix allocate_shared when used with an explicitly convertible allocator

When the allocator is only explicitly convertible from other specializations
of itself, the new version of std::allocate_shared would not work because
it would try to do an implicit conversion. This patch fixes the problem
and adds a test so that we don't fall into the same trap in the future.