[ASan] Only define macros in asan_init_version.h, move the __asan_init declaration back to asan_interface_internal.h

This fixes the issues we've uncovered after landing r212815.

Reviewed at http://reviews.llvm.org/D4500

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@213053 91177308-0d34-0410-b5e6-96231b3b80d8