InternPool: remove memoized_decl

This is neither a type nor a value. Simplifies `addStrLit` as well as
the many places that switch on `InternPool.Key`.

This is a partial revert of bec29b9e498e08202679aa29a45dab2a06a69a1e.

src/codegen/c.zig

@@ -925,7 +925,6 @@ pub const DeclGen = struct {
             .error_set_type,
             .inferred_error_set_type,
             // memoization, not values
-            .memoized_decl,
             .memoized_call,
             => unreachable,
 

src/codegen/llvm.zig

@@ -3796,9 +3796,7 @@ pub const DeclGen = struct {
                     return llvm_union_ty.constNamedStruct(&fields, fields_len);
                 }
             },
-            .memoized_decl,
-            .memoized_call,
-            => unreachable,
+            .memoized_call => unreachable,
         }
     }
 

src/codegen/spirv.zig

@@ -831,9 +831,7 @@ pub const DeclGen = struct {
 
                     try self.addUndef(layout.padding);
                 },
-                .memoized_decl,
-                .memoized_call,
-                => unreachable,
+                .memoized_call => unreachable,
             }
         }
     };