[mlir] return the updated symbol table after inserting into SymbolTable

Inserting a symbol into a SymbolTable may lead to the name of the symbol being
changed in order to ensure uniqueness of symbol names in the table. Return this
new name to spare the caller the need to extract it from the symbol operation.

Depends On D112700

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D112886