[lsan] When detect_leaks=false, be completely silent.

In particular, don't make a fuss if we're passed a malformed suppressions file,
or if we have trouble identifying ld.so. Also, make LSan interface functions
no-ops in this case.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@193108 91177308-0d34-0410-b5e6-96231b3b80d8