Fix clang-tidy shared link with libc++

Currently the ClangTidyMain.cpp fails to link against shared LLVM/Clang libraries
due to the missing symbol:

  clang::tooling::operator<(clang::tooling::Replacement const&,
                            clang::tooling::Replacement const&);

This patch fixes the issue by correctly linking clangToolingCore which contains
the definition.

llvm-svn: 300115