I just learned how to replace newlines with other characters in Emacs. I found Jeremy's blog post from 2007.

I type:

M-x replace-string <return>
C-q C-j
replacement_string_of_choice <return>

C-q is emacs' quoted-insert, which works for inserting control characters;
C-j is the function newline-and-indent which inserts a newline and indents to your major mode.

C-q also works in Xcode as described in this Stack Overflow question