Fix bug where `config.asan_dynamic` in generated ASan `lit.site.cfg` files was...

Fix bug where `config.asan_dynamic` in generated ASan `lit.site.cfg` files was set to `False` for macOS.

`config.asan_dynamic` should actually be `True` because dylibs are the
only supported form of the ASan runtime on Apple platforms.

Reviewers: kubamracek, george.karpenkov, samsonov

Subscribers: srhines, mgorny, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D53183

llvm-svn: 344324