Don't let virtual calls and dynamic casts call Sema::MarkVTableUsed().

Don't let virtual calls and dynamic casts call Sema::MarkVTableUsed().

clang currently calls MarkVTableUsed() for classes that get their virtual
methods called or that participate in a dynamic_cast. This is unnecessary,
since CodeGen only emits vtables when it generates constructor, destructor, and
vtt code. (*)

Note that Sema::MarkVTableUsed() doesn't cause the emission of a vtable.
Its main user-visible effect is that it instantiates virtual member functions
of template classes, to make sure that if codegen decides to write a vtable
all the entries in the vtable are defined.

While this shouldn't change the behavior of codegen (other than being faster),
it does make clang more permissive: virtual methods of templates (in particular
destructors) end up being instantiated less often. In particular, classes that
have members that are smart pointers to incomplete types will now get their
implicit virtual destructor instantiated less frequently. For example, this
used to not compile but does now compile:

    template <typename T> struct OwnPtr {
      ~OwnPtr() { static_assert((sizeof(T) > 0), "TypeMustBeComplete"); }
    };
    class ScriptLoader;
    struct Base { virtual ~Base(); };
    struct Sub : public Base {
      virtual void someFun() const {}
      OwnPtr<ScriptLoader> m_loader;
    };
    void f(Sub *s) { s->someFun(); }

The more permissive behavior matches both gcc (where this is not often
observable, since in practice most things with virtual methods have a key
function, and Sema::DefineUsedVTables() skips vtables for classes with key
functions) and cl (which is my motivation for this change) – this fixes
PR20337.  See this issue and the review thread for some discussions about
optimizations.

This is similar to r213109 in spirit. r225761 was a prerequisite for this
change.

Various tests relied on "a->f()" marking a's vtable as used (in the sema
sense), switch these to just construct a on the stack. This forces
instantiation of the implicit constructor, which will mark the vtable as used.

(*) The exception is -fapple-kext mode: In this mode, qualified calls to
virtual functions (`a->Base::f()`) still go through the vtable, and since the
vtable pointer off this doesn't point to Base's vtable, this needs to reference
Base's vtable directly. To keep this working, keep referencing the vtable for
virtual calls in apple kext mode.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@227073 91177308-0d34-0410-b5e6-96231b3b80d8