[weak vtables] Place class definitions into anonymous namespaces to prevent weak...

[weak vtables] Place class definitions into anonymous namespaces to prevent weak vtables.

This patch places class definitions in implementation files into anonymous
namespaces to prevent weak vtables. This eliminates the need of providing an
out-of-line definition to pin the vtable explicitly to the file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195092 91177308-0d34-0410-b5e6-96231b3b80d8