When marking derived classes' virtual methods ODR-used in order to trigger
instantiation in order to permit devirtualization later in codegen, skip over
pure functions since those can't be devirtualization targets.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175116 91177308-0d34-0410-b5e6-96231b3b80d8