Ignore the "novtable" declspec when not using the Microsoft C++ ABI.

Clang used to silently ignore __declspec(novtable). It is implemented
now, but leaving the vtable uninitialized does not work when using the
Itanium ABI, where the class layout for complex class hierarchies is
stored in the vtable. It might be possible to honor the novtable
attribute in some simple cases and either report an error or ignore
it in more complex situations, but it’s not clear if that would be
worthwhile. There is also value in having a simple and predictable
behavior, so this changes clang to simply ignore novtable when not using
the Microsoft C++ ABI.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@242730 91177308-0d34-0410-b5e6-96231b3b80d8