Fix treatment of symbol versions with unused as-needed libraries.

Fix treatment of symbol versions with unused as-needed libraries.

When we have a weak reference to a symbol defined in an
as-needed library, and that library ends up not-needed, gold
simply clears the version information in the symbol table, even
if the symbol could have been resolved by a needed library later
in the link order. This results in a loss of version information,
which can cause the program to bind to the wrong version at run
time.

This patch lets a dynamic definition override an earlier one if
the earlier one is from a not-needed library, so that we can
retain the version information from the binding to the needed
library. In order to do that, the tracking of needed/not-needed
had to be moved up to symbol resolution time, instead of during
Symbol_table::set_dynsym_indexes().

In cases where we still end up discarding version information,
I've added a warning.

For the original problem report and discussion, see:

https://stackoverflow.com/questions/50751421/undefined-behavior-in-shared-lib-using-libpthread-but-not-having-it-in-elf-as-d

gold/
	* resolve.cc (Symbol_table::resolve): Rename tobinding to
	orig_tobinding.  Call set_is_needed() for objects that resolve
	non-weak references.
	(Symbol_table::should_override): Allow a dynamic definition to
	override an earlier one in a not-needed library.
	* symtab.cc (Symbol_table::set_dynsym_indexes): Remove separate
	processing for as-needed symbols.  Add warning when discarding
	version informatin.
	* testsuite/Makefile.am (weak_as_needed): New test case.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/weak_as_needed.sh: New test script.
	* testsuite/weak_as_needed_a.c: New source file.
	* testsuite/weak_as_needed_b.c: New source file.
	* testsuite/weak_as_needed_b.script: New version script.
	* testsuite/weak_as_needed_c.c: New source file.
	* testsuite/weak_as_needed_c.script: New version script.