[LSan] Ignore all allocations made inside pthread_create.

Thread stack/TLS may be stored by libpthread for future reuse after
thread destruction, and the linked list it's stored in doesn't
even hold valid pointers to the objects, the latter are calculated
by obscure pointer arithmetic.

With this change applied, LSan test suite passes with
"use_ld_allocations" flag defaulted to "false". It still requires more
testing to check if the default can be switched.

llvm-svn: 257975