Make our marking of virtual members functions in a class be

Make our marking of virtual members functions in a class be
deterministic and work properly with templates. Once a class that
needs a vtable has been defined, we now do one if two things:

  - If the class has no key function, we place the class on a list of
    classes whose virtual functions will need to be "marked" at the
    end of the translation unit. The delay until the end of the
    translation unit is needed because we might see template
    specializations of these virtual functions.
  - If the class has a key function, we do nothing; when the key
    function is defined, the class will be placed on the
    aforementioned list.

At the end of the translation unit, we "mark" all of the virtual
functions of the classes on the list as used, possibly causing
template instantiation and other classes to be added to the
list. This gets LLVM's lib/Support/CommandLine.cpp compiling again.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@92821 91177308-0d34-0410-b5e6-96231b3b80d8