Adding GDB PrettyPrinter for mlir::Identifier. This is the first bit from D73546. Primarily setting up the corresponding test. Will add more pretty printers in a separate revision. Reviewed By: dblaikie Differential Revision: https://reviews.llvm.org/D86937 GitOrigin-RevId: 9e9e6e698d8ef5dc5b7576058f2022aab2534a52