Issue 10312 - [CTFE] ICE assigning to const vector decl

https://d.puremagic.com/issues/show_bug.cgi?id=10312

Bringing the fix in #2153 back from the dead.  This change allows gdc to compile the code in issue 10312 - in other words, fixes the frontend ICE.  Someone with backend know-how will need to check if any other difficulties remain with dmd.