sql/ir: use a special character to highlight metavars in templates

[knz]

No description provided.

[cockroach-teamcity]

This change is 

[petermattis]

The use of the special character is interesting, though I'm not sure if the need is compelling enough to justify the usage. A unique prefix would also suffice.

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Comments from Reviewable

[knz]

Do you have a better suggestion?

[petermattis]

Does the template code need to be valid go code? If not, I'd use something like <numRefsPerNode>. If the template code does need to be valid go code, perhaps something like META_numRefsPerNode.

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Review status: 0 of 5 files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable

[justinj]

I can see an argument against having "normal" characters given that metavariables could occur anywhere, including in strings. Does seem like a bit of a pain to work with though... I don't have a suggestion for something that would solve both problems short of moving to a homoiconic language :) so I'll give a weak +1 to using the unicode character, personally.

[knz]

Superseded by #19135.