c++: trivial contracts adjustments

c++: trivial contracts adjustments

gcc/cp/ChangeLog:

	* contracts.h: Tweak comment.
	* contracts.cc: Tweak comment.
	* cp-gimplify.c (cp_genericize_r): Remove commented-out code.
	* decl.c (duplicate_decls): Fix formatting.
	* mangle.c (write_encoding): Update comment.
	* parser.c (cp_parser_conditional_expression): Expand comment.

libstdc++-v3/ChangeLog:

	* src/c++17/contract.cc (__on_contract_violation): Fix formatting.