Type::isObjectType now implements the (more sensible) C++ definition

Type::isObjectType now implements the (more sensible) C++ definition
of "object type" rather than the C definition of "object type". The
difference is that C's "object type" excludes incomplete types such as

  struct X;

However, C's definition also makes it far too easy to use isObjectType
as a means to detect incomplete types when in fact we should use other
means (e.g., Sema::RequireCompleteType) that cope with C++ semantics,
including template instantiation.

I've already audited every use of isObjectType and isIncompleteType to
ensure that they are doing the right thing for both C and C++, so this
is patch does not change any functionality.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@67648 91177308-0d34-0410-b5e6-96231b3b80d8