rust: Add cflag checks against -xc++ and -std.

If -x c++ is passed through a modules cflags, it will be overridden by a
-x c and -std flag added by the build system if the extension is not
.hpp/.hh and cpp_std is not set. This leads to confusing behavior.

Instead, add a helpful error message to guide developers towards the
correct way to specify when a header is  a C++ header and which std
version should be used.

Bug: 171011490
Test: m nothing
Change-Id: I7e7cba504798d47ce1c753ba8699d7475a95095b