[LTO] Fix C++ thunks

[Diego Novillo <dnovillo at google dot com>]

[ CCing jason and mark, to make sure I'm not missing anything
  obvious in the C++ side. ]

This patch is the initial step to make thunks work with LTO.
Essentially, this forces cc1plus to always emit non-asm thunks,
which is a performance problem, but I think I know how to address
it.

Disabling non-asm thunks was not the only problem.  We were also
emitting thunks too late in the pipeline, so they were never
saved to disk.  So, we are now emitting thunks early in
cgraph_optimize instead of doing it during expansion.

This fixes 32 tests in libstdc++ and produces two new failures in
g++ (because non-asm thunks don't support varargs).

We've been chatting offline with Rafael and Ian discussing how to
properly support asm-thunks.  Essentially, cp/method.c:use_thunk
will always emit a new thunk function that contains exactly one
gimple instruction: GIMPLE_THUNK_CALL.

1- GIMPLE_THUNK_CALL has at least four arguments: the thunk, the
   target function, THUNK_FIXED_OFFSET, THUNK_VIRTUAL_OFFSET, and
   in general anything that needs the C++ front end around.

2- Instead of emitting the offset adjusting code in use_thunk, we
   will always emit just the GIMPLE_THUNK_CALL instruction.

The call to GIMPLE_THUNK_CALL is then expanded in
cgraph_expand_thunks, which will be now called *late* in
cgraph_optimize.  This way, we can support emitting thunks out of
lto1 and out of cc1plus.  This is important for this scenario:

$ g++ -flto -c file.cc
$ g++ file.o -o file

file.o will contain two different versions of thunks.  In the IL
sections, it will have the unexpanded GIMPLE_THUNK_CALL code.
And in the final object code, it will have the asm thunks.

This way, if the final link is not done with -flto, the thunks
will be expanded as usual.

Does this sound reasonable?


Thanks.  Diego.