Merge #251: Improved CustomBuildField for std::optional in IPC/libmultiprocess

1977b9f Use std::forward in CustomBuildField for std::optional to allow move semantics, resolves FIXME (Mangoostaa)

Pull request description:

  The function now uses std::forward(value) when building the field, so rvalues are forwarded correctly instead of always being treated as lvalues. This resolves the existing FIXME in that code path.

  Details
  Previously, the implementation forced every optional value to be accessed as an lvalue, which meant rvalues were unnecessarily copied. By applying perfect forwarding, the function now preserves the value category and allows moves where appropriate. The FIXME comment was removed since the intended behavior is now implemented

ACKs for top commit:
  ryanofsky:
    Code review ACK 1977b9f. Thanks for the fix! I'm guessing this change became possible after 3a96cdc from #172 which started to support perfect forwarding

Tree-SHA512: ff67a944aa7402f0652d562057f5a16e1190774a48a06e4f2502dbbedda86e40d4df99113e8a97b73d835bac883bad0705a285968ebd8d2e29e7bc96549f5d5f

Author: ryanofsky

include/mp/type-optional.h

@@ -17,8 +17,7 @@ void CustomBuildField(TypeList<std::optional<LocalType>>,
 {
     if (value) {
         output.setHas();
-        // FIXME: should std::move value if destvalue is rref?
-        BuildField(TypeList<LocalType>(), invoke_context, output, *value);
+        BuildField(TypeList<LocalType>(), invoke_context, output, *std::forward<Value>(value));
     }
 }