Address danalbert's post-commit review comments on D7019 and small fixes.

This patch addresses some comments on http://reviews.llvm.org/D7019.

- Move compiler.py to test/libcxx from test/libcxx/test.
- Make CXXCompiler.target None by default.
- Use `{}` instead of `dict()` to initialize an empty dict.
- Pass the -fsanitize options to both the compile and link commands.

llvm-svn: 226575