hwasan: Untag global variable addresses in tests.

Once we start instrumenting globals, all addresses including those of string literals
that we pass to the operating system will start being tagged. Since we can't rely
on the operating system to be able to cope with these addresses, we need to untag
them before passing them to the operating system. This change introduces a macro
that does so and uses it everywhere it is needed.

Differential Revision: https://reviews.llvm.org/D65768

llvm-svn: 367938