The gcc-in-cxx branch now completes bootstrap

[Ian Lance Taylor <iant at google dot com>]

I'm happy to report that the gcc-in-cxx branch can now bootstrap.  That
is, the code in gcc proper can now be compiled with a C++ compiler.

My plan going forward is as follows (when we are back in stage 1):

* For each difference between trunk and gcc-in-cxx:
  + Try to implement a -Wc++-compat warning which detects the change.
    - If it is possible, implement the warning, and make the changes to
      let gcc bootstrap with the warning.
    - If a warning is not possible for some reason, I will simply
      propose the change by itself.  I expect this will be a small
      subset of the changes, mostly related to the build system and to
      low-level configuration like ansidecl.h.
  + This process will eventually eliminate all differences between trunk
    and gcc-in-cxx, at which point gcc-in-cxx can be retired.

* Implement a configure option, --enable-c++-build or something like
  that, which builds gcc with a C++ compiler.
  + Start running regular builds with that option, to avoid any
    regressions in C++ buildability for cases for which there is no
    -Wc++-compat warning.

* Begin the lobbying process for changing the default value of the
  configure option.

I certainly welcome help on any of these steps.

Ian