Commit 1735f6f53ca19f99c6e9e39496c486af323ba6a8 started to escape all
back-slashes which breaks case when lexicon translations contain substition
and literals with eval-non-safe characters. E.g.  these translations:

"[_1]foo\\n\n" => "[_1]bar\\n\n",
'[_1]foo\n' => '[_1]aÄa\n',

got doubled back-slashes on the maketext() output.

This patch de-escapes escaped backslashes if the literal is compiled as
function argument.

Fixes RT #120457.