[Sanitizer] Fix false positive in snprintf interceptor - take the number of actually written symbols from real snprintf call.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@199899 91177308-0d34-0410-b5e6-96231b3b80d8
[lsan] Expand a comment to document our dynamic TLS hack.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@199900 91177308-0d34-0410-b5e6-96231b3b80d8