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.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@226575 91177308-0d34-0410-b5e6-96231b3b80d8
