[Dlang-internal] Detect CTFE in AssignExp:semantic

[Walter Bright via Dlang-internal]

The way to attack these sorts of problems is to not attempt the complete 
solution as the first step.

Start with a massive simplification - for example, just having mangle() return 
"hello". If that fails in a confusing manner, go even simpler.

Once the very simple solution works, then start adding in the complex solution 
bit by bit, ensuring each piece works before adding the next.