Fix a bug/missing-feature Ted noticed: the 'unused' warning should not

warn about the last stmt in a stmtexpr, f.e. there should be no warning for:

int maxval_stmt_expr(int x, int y) {
  return ({int _a = x, _b = y; _a > _b ? _a : _b; });
}

llvm-svn: 41655