[lsan] Handle fork() correctly.

Update the main thread's os_id on every pthread_create, and before
initiating leak checking. This ensures that we have the correct os_id even if we
have forked after Init().

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@185815 91177308-0d34-0410-b5e6-96231b3b80d8