[lit][tests]Add feature libcxx-used and use it in llvm-*-fuzzer tests

When a LLVM binary such as llvm-*-fuzzer is built with libc++, it has dependency on libc++. The path to find shared libraries specified in llvm-*-fuzzer is relative. As a result, these binaries cannot be copied to an arbitrary directory and launched from there. Changes in this patch add a LIT feature to indicate that libc++ is used to build and, based on the feature exclude test cases that test by copying llvm-*-fuzzer binaries to a directory.

Reviewers: hubert.reinterpretcast, dberris, amyk, jasonliu, EricWF

Reviewed By: hubert.reinterpretcast, amyk

Subscribers: javed.absar, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61265

llvm-svn: 360672