[ASan] Make sure none of the __asan_gen_ global strings end up in the symbol table, add a test.

This should fix http://llvm.org/bugs/show_bug.cgi?id=17976
Another test checking for the global variables' locations and prefixes on Darwin will be committed separately.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198017 91177308-0d34-0410-b5e6-96231b3b80d8