stage2: improve Decl lifetime management

 * Compilation: iteration over the deletion_set only tries to delete the
   first one, relying on Decl destroy to remove itself from the deletion
   set.
 * link: `freeDecl` now has to handle the possibility of freeing a Decl
   that was never called with `allocateDeclIndexes`.
 * `deleteDecl` recursively iterates over a Decl's Namespace sub-Decl
   objects and calls `deleteDecl` on them.
   - Prevents Decl objects from being destroyed when they are still in
     `deletion_set`.
 * Sema: fix cleanup of anonymous Decl objects when an error occurs
   during semantic analysis.
 * tests: update test cases for fully qualified names

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