[lit] Support the `%shared_libasan` lit substitution on Apple platforms.

Summary:
The previous value looks Linux specific so that has been guarded with
the host OS being Linux.

On Apple platforms `%shared_libasan` expands to the absolute path of the
ASan dylib.

Previously on Linux `%shared_libasan` expanded to just the file name
of the shared library rather than the absolute path to the library.
This is likely a bug because it would rely on the OS's dynamic linker
to find the shared library which could accidentally pick up a system copy
rather than the shared library that was just built.

For other platforms we emit a warning if `config.asan_dynamic` is true.

This patch also only defines the substitution when `config.asan_dynamic`
is true because using this substitution only makes sense when the
dynamic library is available.

Reviewers: kubamracek, george.karpenkov, mgorny, phosek, etienneb, samsonov, kcc

Subscribers: #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D53111

llvm-svn: 344434