[msan] Don't delete MSanAtExitRecord

Summary:
Pre 2.27 libc can run same atexit handler twice
We will keep MSanAtExitRecord and reset fun to mark it as executed.

Fix PR40162

Reviewers: eugenis

Subscribers: jfb, jdoerfert, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D58221

llvm-svn: 354005