lto-plugin - support -save-temps, -v, --version.

lto-plugin - support -save-temps, -v, --version.

This patch makes the lto-plugin follow the same approach
to save-temps as collect2.

-save-temps causes the temp file to be named meaningfully,
and for the relevant input files to be saved in CWD.

-v, —version causes the save actions to be output to stderr.

one can get this to happen by just putting -save-temps, -v on
the regular link line or (for compatibility with the way the
 -debug flag works) by appending -plugin-opt=-save-temps, etc.

lto-plugin/

2019-05-15  Iain Sandoe  <iain@sandoe.co.uk>

	* lto-plugin.c (exec_lto_wrapper): Make the wrapper
	arguments filename more user-friendly.
	(file_exists, maybe_unlink): New.
	(cleanup_handler): Use maybe unlink to handle the
	case when temps should be saved.
	(process_option): Look for -v, —-version, -save-temps.
	(onload): Record the linker output file name.
	Check for -v, —-version, -save-temps in the GCC collect
	options environment.

From-SVN: r271202