Update gn to 468c6128db7fabe32a29d4753460ef53594406fc

[cbracken]

In order to, at some future date, support building Swift in the engine, we'll need an updated version of gn.

Note that all versions of gn after
f792b9756418af8ab8a91a4c15b582431cb86ff9 up to and including this version are affected by a crasher when building Swift source_set targets. The workaround to support Swift is to opt back in to stamp files until the issue is fixed.

• Bug: https://crbug.com/gn/380799198
• Discussion: http://shortn/_KbylT3nDsZ

Either way, prior to this patch, we were using a version of gn from 2021, so we should probably update to a more recent version of gn regardless.

Issue: flutter/flutter#144791

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[zanderso]

New gn seems to have different ideas about formatting, but that's no biggie. More mysterious is the clangd failure. That might be related to the build that timed out: If the new gn changed the clang command line, that could both confuse clangd and invalidate the RBE cache.