Another shot at fixing the issues with (constant) struct literals and their addresses. See DMD2682, #218, #324.

The idea is to separate the notion of const from 'this variable can always be
replaced with its initializer' in the frontend. To do that, I introduced
Declaration::isSameAsInitializer, which is overridden in VarDeclaration to
return false for constants that have a struct literal initializer.

So
{{{
const S s = S(5);
void foo() { auto ps = &s; }
// is no longer replaced by
void foo() { auto ps = &(S(5)); }
}}}

To make taking the address of a struct constant with a struct-initializer
outside of function scope possible, I made sure that AddrExp::optimize doesn't
try to run the argument's optimization with WANTinterpret - that'd again
replace the constant with a struct literal temporary.

dmd/declaration.h

@@ -135,6 +135,8 @@ struct Declaration : Dsymbol
     int isParameter()    { return storage_class & STCparameter; }
     int isDeprecated()   { return storage_class & STCdeprecated; }
     int isOverride()     { return storage_class & STCoverride; }
+    
+    virtual int isSameAsInitializer() { return isConst(); };
 
     int isIn()    { return storage_class & STCin; }
     int isOut()   { return storage_class & STCout; }
@@ -282,6 +284,8 @@ struct VarDeclaration : Declaration
     void checkCtorConstInit();
     void checkNestedReference(Scope *sc, Loc loc);
     Dsymbol *toAlias();
+    
+    virtual int isSameAsInitializer();
 
 #if IN_DMD
     void toObjFile(int multiobj);			// compile to .obj file