This enhances the symbol table utility methods to handle the case where an unknown operation may define a symbol table. When walking symbols, we now collect all symbol uses before allowing the user to iterate. This prevents the user from assuming that all symbols are actually known before performing a transformation.
PiperOrigin-RevId: 273651963
MLIR uses symbol references to model references to many global entities, such as functions/variables/etc. Before this change, there is no way to actually reason about the uses of such entities. This change provides a walker for symbol references(via SymbolTable::walkSymbolUses), as well as 'use_empty' support(via SymbolTable::symbol_use_empty). It also resolves some deficiencies in the LangRef definition of SymbolRefAttr, namely the restrictions on where a SymbolRefAttr can be stored, ArrayAttr and DictionaryAttr, and the relationship with operations containing the SymbolTable trait.
PiperOrigin-RevId: 273549331
