[dmd-internals] Six more regression and ICE patches

[Don Clugston]

// 3273 and 3602 have made DbC practically unusable.
// 4768 is a D1-only wrong-code regression
// 4869 is just nasty

3273 Regression(2.031) struct invariant + dtor fails to compile (no line number)
3665 Regression(1.051, 2.036) Assignment with array slicing does not work
4768 Regression(1.056): wrong code with forward declaration of enum
3602 ICE(tocsym.c) compiling a class, if its super class has preconditions
4869 auto return + inheritance + modules = compiler crashes(toctype.c)
4465 ICE(symbol.c): immutable type inference with ^^2