[MLIR] Use nested symbol to identify kernel in `LaunchFuncOp`.

[MLIR] Use nested symbol to identify kernel in `LaunchFuncOp`.

Summary:
Use a nested symbol to identify the kernel to be invoked by a `LaunchFuncOp` in the GPU dialect.
This replaces the two attributes that were used to identify the kernel module and the kernel within seperately.

Differential Revision: https://reviews.llvm.org/D78551