tree-inline: Fix TREE_READONLY of parameter replacements

tree-inline: Fix TREE_READONLY of parameter replacements

tree-inline leaves behind VAR_DECLs which are TREE_READONLY (because
they are copies of const parameters) but are written to because they
need to be initialized.  This patch resets the flag unconditionally so
that this does not happen.

There are other sources of variables which are incorrectly marked as
TREE_READOLY, but with this patch and a verifier catching them I can
at least compile the Ada run-time library.

gcc/ChangeLog:

2021-06-22  Richard Biener  <rguenther@suse.de>
	    Martin Jambor  <mjambor@suse.cz>

	* tree-inline.c (setup_one_parameter): Set TREE_READONLY of the
	param replacement unconditionally.  Adjust comment.