Fix incorrect regeneration for IDL targets

[OneBlue]

Summary of the Pull Request

#40130 introduced a regression that caused the idl targets to always be considered out of date. This changes solves this by explicitly setting the target stamp file and fixing the output of the dlldata file to only one command

PR Checklist

• Closes: Link to issue #xxx
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated if needed and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated if needed
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

[Copilot]

Pull request overview

Fixes a build regression where IDL-related CMake targets are always considered out-of-date, impacting incremental builds across dependent Windows components.

Changes:

• Tracks dlldata_<platform>.c as an output from only the final proxy-IDL MIDL invocation to avoid conflicting/duplicated outputs.
• Suppresses unwanted MIDL byproducts for “no proxy” IDLs by redirecting /iid, /proxy, and /dlldata outputs to nul.
• Attempts to add a stamp-touch step for Visual Studio incremental up-to-date tracking on the IDL custom target.

[benhillis]

LGTM - thanks for cleaning up my mess ;)