No longer reject tag declarations in the clause-1 of a for loop.

We currently reject this valid C construct by claiming it declares a
non-local variable: for (struct { int i; } s={0}; s.i != 0; s.i--) ;

We expected all declaration in the clause-1 declaration statement to be
a local VarDecl, but there can be other declarations involved such as a
tag declaration. This fixes PR35757.