Generate and scan documentation output in Ddoc tests.

Generate and scan documentation output in Ddoc tests.

The tests in gdc.test/compilable/ddoc*.d don't require the module to be
compiled all the way down to object code.  Instead, only compile the
test sources with -fdoc, and scan the generated html content.

gcc/testsuite/ChangeLog:

	PR d/88039
	* gdc.test/gdc-test.exp (gdc-convert-args): Handle -D.
	(dmd2dg): Check generated html in ddoc tests.
	(gdc-do-test): Set dg-do-what-default to compile for ddoc tests.

From-SVN: r266933