[mlir] Allow SymbolTable to update existing symbols

[tpopp]

Summary

Previous behavior would fail if inserting an operation that already
existed. Now SymbolTable::insert can also be used as a way to make a
symbol's name unique even after insertion.

Further TODOs have been left over naming and consistent behavior
considerations.

[tpopp]

Correct typo and add comment

[rriddle]

Nice!

mlir/lib/IR/SymbolTable.cpp

153–154

Can you update the doc here and in the header? It should probably say that it only inserts into the symbol table operation if it isn't already a child.

157–158

All of this can be wrapped into the if given that it is only useful when inserting the operation.