Replace the use of C++ standard library in libFuzzer

[petrhosek]

libFuzzer implementation uses C++ standard library but that introduces a number of issues:

• When libFuzzer is built against libc++, but the application uses libstdc++, we cannot use the two together.
• When libc++ itself is instrumented (for example with ASan), we cannot use it for libFuzzer.
• When the application is implemented in C, we may want to avoid a dependency on the C++ library.

The current solution is to use a private version of libc++, which was introduced in D37631, but this introduces additional complexity and overhead to the build (since we need to build libc++ twice).

We should consider replacing the use of C++ standard library altogether and instead use custom implementation of the necessary containers and other utilities.

libFuzzer primarily uses std::string, std::vector, std::unordered_map and std::set. Rather than reimplementing these ourselves, we should be able to use stripped down and simplified implementations from LLVM ADT, which is strategy which was also adopted for sanitizers.

[petrhosek]

See #70407 for an example of issues that users run into when building the private version of libc++.

[vitalybuka]

Time to time I wish we do an opposite: find a way to use private libc++ in sanitizers.