[sanitizer] Avoid memset call in tsan

Summary: Assigning an array of bools to {false, false, false} can, in certain build configurations, lead to a memset call. Use internal_memset to avoid this.

Reviewers: eugenis

Subscribers: kubamracek, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D56107

llvm-svn: 350098