[lsan] Add __lsan_disable() and __lsan_enable().

Objects allocated after a call to __lsan_disable() will be treated as
live memory. Also add a ScopedDisabler.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@183099 91177308-0d34-0410-b5e6-96231b3b80d8