[MLIR] Extend Symbol verification to reject public symbol declarations.

• Extend the Symbol interface with isDeclaration to identify operations that declare a symbol as opposed to define it.
• Extend verification to disallow public declarations as per the discussion in https://llvm.discourse.group/t/rfc-symbol-definition-declaration-x-visibility-checks/2140
• Adopt the new interface for FuncOp and fix test and code to not have/create public function declarations.

Differential Revision: https://reviews.llvm.org/D91456