Add empty virtual destructor...

... as this otherwise triggers -Wnon-virtual-dtor.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@190520 91177308-0d34-0410-b5e6-96231b3b80d8
Revert r190520 as it wasn't the right fix.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@190524 91177308-0d34-0410-b5e6-96231b3b80d8