This PR is only meant to change Darwin link line with -fsanitize=address. With this PR, I can use addresssanitizer successfully (and easily) on my Mac.
The PR finds a libldc_rt.asan_osx_dynamic.dylib or libclang_rt.asan_osx_dynamic.dylib in LDC's lib dir, and falls back to the old behavior (passing -fsanitize=address to the linker driver) if nothing is found. So now the user can copy his self-built library to LDC's lib dir and be happy :-)
(the cmake script could copy+rename the library to the lib dir when the library is found in the LLVM binaries)
