a8ffba2c cleared the persistent symbol
table caches in ~PersistentSymbolTable() to work around an
AddressSanitizer crash when a non-empty imports cache is destroyed after
the cache repository it references.

The constructor of the PersistentSymbolTable singleton creates a
PersistentSymbolTableRepo function-local static, then a
RecursiveImportCacheRepository function-local static. The order of
destruction of these 3 static objects is the reverse of the order of
their construction completion:
~PersistentSymbolTable()
recursiveImportCacheRepositoryObject's destruction
~PersistentSymbolTableRepo()

Since PersistentSymbolTableRepo::importsCache depends on
recursiveImportCacheRepositoryObject and not vice versa, a proper fix is
to create the cache repository before and thus destroy after the
persistent symbol table repository.