[Performance] Precalculating AST during save freezes UI

When saving a java file, the post save actions may trigger the calculation of the AST in the UI thread, which sometimes results in a UI freeze.

Here's an example:

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The code that does this is:

https://github.com/eclipse-jdt/eclipse.jdt.ui/blob/ef74c4430d96c065c7785c7eb4b6835517a080b2/org.eclipse.jdt.ui/core%20extension/org/eclipse/jdt/internal/corext/fix/CleanUpPostSaveListener.java#L377-L379

Start in org.eclipse.jdt.ui/core extension/org/eclipse/jdt/internal/corext/fix/CleanUpPostSaveListener.java at lines 377-379 and trace the post-save path that precalculates the AST. Reproduce the freeze when saving a Java file, then verify that AST calculation no longer blocks the UI thread and that existing post-save behavior still works.

Written by the indexing model from the issue text.