(1/2) Deprecate -gc in favour of -g

Start the deprecation process of ``-gc``.  There is almost no difference now between -g and -gc other than.

1. On MSCOFF/OMF targets, ``dchar`` is emitted as ``ulong``
2. On ELF/DWARF targets, ``DW_LANG_C89`` is emitted instead of ``DW_LANG_D``.

I would strongly suggest that all known bugs in ``-g`` be fixed before pushing this.  For DWARF, this means that if you can't reproduce the debugging issue in ``-gc``, then it probably isn't a (sic) DMD problem, and needs to be fixed in GDB.

**KNOWN BUGS EXTERNAL TO DMD**
- https://d.puremagic.com/issues/show_bug.cgi?id=4181


Related pull request #3083