[libFuzzer] Avoid STL in MSan test.

Summary:
STL can cause MSan false positives if lib[std]c++ isn't instrumented
with MSan.

Reviewers: kcc

Reviewed By: kcc

Subscribers: Dor1s, llvm-commits

Differential Revision: https://reviews.llvm.org/D49404

llvm-svn: 337224