docs: Copy Block-ABI-Apple.txt to output.

This fixes the URL permanence of that URL. This is a bit of a hack. See
the FIXME in the patch for what the "real" solution should be.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171795 91177308-0d34-0410-b5e6-96231b3b80d8

docs/Makefile.sphinx

@@ -50,6 +50,10 @@ html:
 	@# Kind of a hack, but HTML-formatted docs are on the way out anyway.
 	@echo "Copying legacy HTML-formatted docs into $(BUILDDIR)/html"
 	@cp -a *.html $(BUILDDIR)/html
+	@# FIXME: What we really need is a way to specify redirects, so that
+	@# we can just redirect to a reST'ified version of this document.
+	@# PR14714 is tracking the issue of redirects.
+	@cp -a Block-ABI-Apple.txt $(BUILDDIR)/html
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."
 
 dirhtml: