[msan] Intercept qsort, qsort_r.

Summary:
This fixes qsort-related false positives with glibc-2.27.
I'm not entirely sure why they did not show up with the earlier
versions; the code seems similar enough.

Reviewers: vitalybuka

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D71740