lto: fix error handling for -Wl,-plugin-opt=debug

lto: fix error handling for -Wl,-plugin-opt=debug

When one uses something like: -Wl,-plugin-opt=debug,
we end up with lto1 WPA invocation that has 'debug'
on command line. We interpret that as input filename.

The patch moves resolution checking later so that we end up with
a reasonable error message:

lto1: fatal error: open debug failed: No such file or directory
compilation terminated.

	PR lto/104333

gcc/lto/ChangeLog:

	* lto-common.cc (read_cgraph_and_symbols): Move resolution
	checking for number of files later and report a reasonable
	error message.
	* lto-object.cc (lto_obj_file_open): Make error fatal.