Centralize error reporting of improper uses of incomplete types in the

Centralize error reporting of improper uses of incomplete types in the
new DiagnoseIncompleteType. It provides additional information about
struct/class/union/enum types when possible, either by pointing to the
forward declaration of that type or by pointing to the definition (if
we're in the process of defining that type).
Fixes <rdar://problem/6500531>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@62521 91177308-0d34-0410-b5e6-96231b3b80d8