[clang][Sema] Split a err_typecheck_assign_const diagnostic into a separate tablegen entry (#179895)

As of recently, in LLDB, when trying to mutate an object in a const
method, we emit a hint about why we failed to run the expression (with
an associated hint on how to fix it). This relies on the diagnostic ID
that Clang told us about. However, we only want to emit this message
when we assign to a member in a const method. But not all the other
situations that `err_typecheck_assign` gets used in. We currently work
around this by grepping the error message, but it would be nice if we
could just rely on the diagnostic ID.

This patch splits out the relevant diagnostic.

This isn't urgent and we can live with the "grep the error message"
approach. But if the Clang maintainers don't feel strongly about keeping
the tablegen as-is, it'd be nice to clean up from LLDB's perspective.

GitOrigin-RevId: 9c9fff983fff3b92406065e33468234b28278b11