File name issues reprise [3.0 regression]

[Neil Booth <neil at daikokuya dot demon dot co dot uk>]

In debugging ObjC regressions, I saw the same line numbering issues
(i.e. right line, wrong file) in objc-parse.c that the C++ guys are
seeing in via some of their headers.

So I think this is an issue in c-lex.c.  The problem seems to be that
there are various variables keeping track of filenames and
linenumbers, and it's not clear to me what exactly they all mean, nor
that they are being updated correctly in the FC_RENAME case (which is
caused by #line directives, like happens in objc-parse.c a lot, being
a generated file).

Indeed, c-lex.c's handler doesn't handle FC_RENAME especially at all.
I suspect the issue may have something to do with toplev.c's
push_src_loc functions not doing anything for #line's (FC_RENAME).

I'd appreciate it if someone with more debugging symbol knowledge than
me took a look.  IMO this is a serious regression in 3.0 - it makes
debugging some things near impossible.

Neil.