Remove a couple of memset usages from TSan, introduced in r288624.

TSan runtime shouldn't contain memset, so internal_memset is used
instead and syntax that emits memset is avoided.

This doesn't fail in-tree due to TSan being build with -03, but it fails
when TSan is built with -O0, and is (I think) a true positive.

Patch by Sam McCall, review: https://reviews.llvm.org/D27407

llvm-svn: 288672