create field decl objects for the members of a struct/union. Diagnose code

like:
struct S { struct S {} X; };

with:
t.c:2:19: error: nested redefinition of 'struct'
struct S { struct S {} X; };
                  ^
t.c:2:1: error: previous definition is here
struct S { struct S {} X; };
^

llvm-svn: 39292