- Removes SYMROOT from the Generated.xcconfig. Having it causes current
  versions of Xcode to switch the project's build output to "Legacy",
  which causes anything not overridden to use a project-relative build
  directory instead of a shared directory in DerivedData, breaking
  anything with subprojects that it depends on.
  This means that `flutter run` and builds from Xcode will use
  completely different build directories, but that each should be
  internally consistent.
- Moves the FlutterMacOS.framework to $SRCROOT/Flutter. This is
  consistent with the approach we're moving to for all desktop
  platforms, and avoids issues finding it now that SYMROOT doesn't match
  for the two different build modes.

Fixes #32494