Fix ASAN build with older compiler-rt versions.

compiler-rt recently added the __asan_handle_no_return() function that libc++abi
needs to use, however older versions of compiler-rt don't declare this interface
publicly and that breaks the libc++abi build.

This patch attempts to fix the issues by declaring the asan function explicitly,
so we don't depend on compiler-rt to provide the declaration.

llvm-svn: 313308