[llvm] StackProtector: use isInTailCallPosition to verify tail call position (PR #68997)

[Fangrui Song via llvm-commits]

================
@@ -0,0 +1,14 @@
+; RUN: llc -mtriple=x86_64-linux-gnu %s -o - 2>&1 | FileCheck %s
----------------
MaskRay wrote:

Merge this into `tailcc-ssp.ll`?

https://github.com/llvm/llvm-project/pull/68997