[lit] Disable test on darwin when building shared libs.

Summary:
This test fails to link shared libraries because tries to run
a copied version of clang-check to see if the mock version of libcxx
in the same directory can be loaded dynamically.  Since the test is
specifically designed not to look in the default just-built lib
directory, it must be disabled when building with
BUILD_SHARED_LIBS=ON.

Currently only disabling it on Darwin and basing it on the
enable_shared flag.

Reviewed By: ilya-biryukov

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D61697

llvm-svn: 363298