The motivation is to fix a crash on

  struct S {} s;
  Foo S::~S() { s.~S(); }

What was happening here was that S::~S() was marked as invalid since its
return type is invalid, and as a consequence CheckFunctionDeclaration() wasn't
called and S::~S() didn't get merged into S's implicit destructor.  This way,
the class ended up with two destructors, which confused the overload printer
when it suddenly had to print two possible destructors for `s.~S()`.

In addition to fixing the crash, this change also seems to improve diagnostics
in a few other places, see test changes.

Crash found by SLi's bot.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@229639 91177308-0d34-0410-b5e6-96231b3b80d8