Add a C++ wrapper for GCC C plug-in

Add a C++ wrapper for GCC C plug-in

This patch introduces a new class which wraps the GCC C compile plug-in.
It is a little unfortunate that this all happened in between the time that
GCC moved to C++ and GDB moved to C++, leaving us with an ABI promise to
support a C-like interface.  The hope is to isolate GDB from some of this
should it change in the future.

Broadly, what this does is replace calls like:

  C_CTX (context)->c_ops->operation (C_CTX (context), ...);

with calls that now look like:

  context->c_plugin->operation (...);

This API will be further refined in following patches when struct
compile_instance/compile_c_instance are changed into classes.

gdb/ChangeLog:
        * Makefile.in (HFILES_NO_SRCDIR): Add compile/gcc-c-plugin.h.
        * compile/compile-c-types.c: Define GCC_METHODN macros and include
        gcc-c-fe.def to define C plugin.
        (delete_instance): Delete `c_plugin'.
        (new_compile_instance): Initialize `c_plugin'.
        * compile/compile-c.h: Include gcc_c_plugin.h.
        (struct compile_c_instance) <c_plugin>: New member.
        * gcc-c-plugin.h: New file.
        Update all callers with API change.