Limit the template instantiation depth to some user-configurable value

(default: 99). Beyond this limit, produce an error and consider the
current template instantiation a failure.

The stack we're building to track the instantiations will, eventually,
be used to produce instantiation backtraces from diagnostics within
template instantiation. However, we're not quite there yet.

This adds a new Clang driver option -ftemplate-depth=NNN, which should
eventually be generated from the GCC command-line operation
-ftemplate-depth-NNN (note the '-' rather than the '='!). I did not
make the driver changes to do this mapping.

llvm-svn: 66513