Backports Ddoc fix that is present in upstream, but did not make its way
into the C++ port of the D front-end implementation.

The old special types for C long, unsigned long, and long double have
also been removed as neither the compiler nor druntime bindings support
handling it anymore.

Commits merged from dmd.

	Backport Issue 14633: Fixed false DDoc warnings
	https://github.com/dlang/dmd/pull/9027

	Remove old support code for struct __c_long/ulong/long_double
	https://github.com/dlang/dmd/pull/9028

From-SVN: r266719