Android.rules: Use libc++ by default

libstdc++ will soon be dropped from the android NDK. This patch makes
sure we are prepared for that by using libc++ in tests by default (i.e.,
except for libstdc++ data formatter tests).

Only a couple of small tweaks were needed to make this work:
- Add the libc++ include paths to CXXFLAGS only. This was necessary to
  make the tests compile with -fmodules. The modules tests have been
  disabled, but this way, they will be ready for them if they are
  enabled.
- in one test I had to add an explicit std::string copy to make sure the
  copy constructor is there for the expression evaluator to find it.

llvm-svn: 335344