[dfsan] Introduce dfsan_read_label runtime function.

Differential Revision: http://llvm-reviews.chandlerc.com/D1349

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@188319 91177308-0d34-0410-b5e6-96231b3b80d8

Conflicts:

	lib/dfsan/lit_tests/basic.c

Change-Id: I00579ef18f5597e59d84d3c999a903ec61e74a2c