This is a case where bogus #line directives can confuse libcpp into 
producing nonsensical line numbers, even leading to a crash later on in LTO.

The following patch moves the test earlier to a point where we can more 
easily recover from the error condition. I should note that I changed 
the raw fprintf (stderr) to a cpp_error call, which is a slight change 
in behaviour (we don't even get to LTO anymore due to erroring out earlier).

Bootstrapped and tested on x86_64-linux (as always including Ada, which 
failed with an earlier version of the patch). Ok?


Bernd