[C++, test] Fix typo in NSS* vars

The NSS FileCheck variables at the end of the
CodeGenCXX/split-stacks.cpp clang testcase are off by 1, resulting in
the use of an undefined variable (NSS3). One of the CHECK-NOT is also
redundant because _Z8tnosplitIiEiv uses the same attribute as _Z3foov
without split stack. This commit fixes that.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D99839