[lsan] Avoid segfaults during threads destruction under high load

This patch addresses two issues:

	* It turned out that suspended thread may have dtls->dtv_size == kDestroyedThread (-1)
	and LSan wrongly assumes that DTV is available. This leads to SEGV when LSan tries to
	iterate through DTV that is invalid.
	* In some rare cases GetRegistersAndSP can fail with errno 3 (ESRCH). In this case LSan
	assumes that the whole stack of a given thread is available. This is wrong because ESRCH
	can indicate that suspended thread was destroyed and its stack was unmapped. This patch
	properly handles ESRCH from GetRegistersAndSP in order to avoid invalid accesses to already
	unpapped threads stack.

Differential Revision: https://reviews.llvm.org/D30818

llvm-svn: 299630