[lsan] Do not check for leaks in the forked process

Summary:
If calling process had threads then forked process will fail to detect
references from them.

Fixes https://github.com/google/sanitizers/issues/836

Reviewers: alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47751

llvm-svn: 334036