[TSan] Try harder to avoid compiler-generated memset calls.

check_memcpy test added in r254959 fails on some configurations due to
memset() calls inserted by Clang. Try harder to avoid them:
* Explicitly use internal_memset() instead of empty braced-initializer.
* Replace "new T()" with "new T", as the former generates zero-initialization
  for structs in C++11.

llvm-svn: 255136