If `\cL` is defined as `\mathcal{L}`, and `\til` as `\tilde{#1}`,
then `\til\cL` should expand to `\tilde{\mathcal{L}}`, but pandoc
was expanding it to `\tilde\mathcal{L}`. This is fixed by
parsing the arguments in "verbatim mode" when the macro expands
arguments at the point of use.
Closes#6796.
