add type attribute warn_unused, for -Wunused-variable warnings (pr#14253)

The functionality is equivalent to the GCC attribute. Variables of tagged
types will be warned about as unused if they are not used in any way
except for possible (even non-trivial) ctors/dtors called. Useful for tagging
classes like std::string (which is not part of this commit).

llvm-svn: 186765