Rename llvm::markup namespace to swift::markup

This was naming was cargoed from long ago and this functionality isn't
directly related to LLVM, it's specific to Swift.

tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp

@@ -743,7 +743,7 @@ static bool passCursorInfoForDecl(const ValueDecl *VD,
           llvm::raw_svector_ostream OSBuf(Buf);
           SwiftLangSupport::printDisplayName(RelatedDecl, OSBuf);
         }
-        llvm::markup::appendWithXMLEscaping(OS, Buf);
+        swift::markup::appendWithXMLEscaping(OS, Buf);
       }
       OS<<"</RelatedName>";
     }