Fix fix<N> when variable templates are not supported.
There were some typos that checked EIGEN_HAS_CXX14 that should have
checked EIGEN_HAS_CXX14_VARIABLE_TEMPLATES, causing a mismatch
in some of the Eigen::fix<N> assumptions.
Also fixed the symbolic_index test when
EIGEN_HAS_CXX14_VARIABLE_TEMPLATES is 0.
Fixes #2308 (closed)