Copying symbol type in ld script assignments

Copying symbol type in ld script assignments

There is a call to update_definedness between code that evaluates an
assignment expression value and code that transfers symbol
attributes.  When script assignment expressions contain DEFINED, that
can mean the wrong symbol type is copied.  This patch tracks symbols
read during expression evaluation, rather than examining the
expression and re-evaluating conditionals.  Not only does this
simplify the code, it also means ld can now copy symbol types in more
complex expressions.

An unfortunate side effect of copying symbol type for more complex
expressions affects mmix, which uses
 PROVIDE (Main = DEFINED (Main) ? Main : (DEFINED (_start) ? _start : _start.));
in a default script.  So now _start or _start. symbol type may be
copied, losing the function type specially set up for Main.  This can
be avoided by making bfd_copy_link_hash_symbol_type do nothing for
mmix.

bfd/
	* elf64-mmix.c (bfd_elf64_bfd_copy_link_hash_symbol_type): Define.
ld/
	* ldexp.h (struct ldexp_control): Add "assign_src".
	* ldexp.c (fold_trinary): Save and restore assign_src around
	condition evaluation.
	(fold_name <NAME>): Set expld.assign_src.
	(try_copy_symbol_type): Delete.
	(exp_fold_tree_1): Set symbol type using expld.assign_src.