Give unit test its own LLVMContext so MDNodes aren't leaked even if we never call llvm_shutdown.

Found by valgrind.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194797 91177308-0d34-0410-b5e6-96231b3b80d8
Redirect unused test case output to /dev/null

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194798 91177308-0d34-0410-b5e6-96231b3b80d8
llvm-cov: Clean up memory leaks.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194799 91177308-0d34-0410-b5e6-96231b3b80d8
[asan] use GlobalValue::PrivateLinkage for coverage guard to save quite a bit of code size

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194800 91177308-0d34-0410-b5e6-96231b3b80d8