Fix bug in constant evaluation exposed by 176.gcc.

 - Evaluation of , operator used bogus assumption that LHS could be
   evaluated as an integral expression even though its type is
   unspecified.

This change is making isICE very permissive of the LHS in non-evaluated 
contexts because it is not clear what predicate we would use to reject 
code here. The standard didn't offer me any guidance; opinions?

llvm-svn: 59196