Don't search for third party libraries while using MSan

Don't search for third party libraries while using MSan

On the average user's system, those libraries will not be compiled with
MSan. Prior to this change, the LLVM test suite was full of false
positives from calls from third party libraries to MSan interceptors
like strlen.

We can remove this check if MSan ever grows a suppression mechanism
similar to TSan's.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253526 91177308-0d34-0410-b5e6-96231b3b80d8