[sanitizer] Don't miss threads by ThreadSuspender

Summary:
Enumerating /proc/<pid>/task/ dir Linux may stop if thread is dead. In this case
we miss some alive threads and can report false memory leaks.
To solve this issue we repeat enumeration if the last thread is dead.
Do detect dead threads same way as proc_task_readdir we use
/proc/<pid>/task/<tid>/status.

Similarly it also ends enumeration of if proc_fill_cache fails, but in this case
Linux sets inode to 1 (Bad block).

And just in case re-list threads if we had to call internal_getdents more than
twice or result takes more than half of the buffer.

Reviewers: eugenis, dvyukov, glider

Subscribers: llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D46517

llvm-svn: 331953