[Bug debug/109805] LTO affecting -fdebug-prefix-map

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109805

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
Created attachment 55064
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55064&action=edit
prototype

This works for me.  The consistency check is not fully implemented and instead
of passing down no -fdebug-prefix-map the patch passes the first but warns:

> ./xgcc -B. t.o t2.o -o t               
lto-wrapper: warning: option
-fdebug-prefix-map=/home/rguenther/obj-trunk-g/gcc=/bbb with different values,
using /home/rguenther/obj-trunk-g/gcc=/aaa

to make consistency checking work we need to record -fcanon-prefix-map
and the full set of -f{file,debug}-prefix-map options in order (I think
file and debug variants can be considered the same) of the first TU and
compare that to each of the following TUs.

As implemented we only diagnose mismatches of options that are actually
given (because we don't stream the option when not given).

We could also emit a hard error when there's a mismatch.

Note a link-time specified option will simply ignore all options from the
compile-time (but only for the link-time unit, the compile-time debug info
has already been generated with the originally specified options).

Not sure what the best behavior is here, any input appreciated.