The Xcode 9 developer beta recompiles everything when switching between building from the Xcode IDE and building from the command line.
Created attachment 312566 [details]
Note that there are important steps to take when updating ANGLE. See http://trac.webkit.org/wiki/UpdatingANGLE
Fixed in <https://trac.webkit.org/r218056>.
(In reply to mitz from comment #3)
> Fixed in <https://trac.webkit.org/r218056>.
Reverted in <https://trac.webkit.org/r218057> because it made the Xcode IDE reindex everything each time the workspace was opened.
Created attachment 317885 [details]
Fixed in <https://trac.webkit.org/r220563>.