CTFE: fix segfault after ICE in DotVarExp

If an ICE occurs in DotVarExp, it would segfault when trying to dereference
EXP_CANT_INTERPRET.
CTFE: DotTypeExp cannot be interpreted

Generate an ICE if it tries, rather than an incomprehensible error.
CTFE: Don't save local variables in the global constants list

Even if they are immutable local variables (eg, bug 8400)
Merge pull request #2105 from donc/ctfe_internal_fixes

Ctfe internal fixes