Require compile-time constant format strings to `shellify`

Require compile-time constant format strings to `shellify`

Summary:
Because it's impossible to require that a parameter to a function is a compile-time constant string this replaces `shellify()` with the user-defined-literal suffix `_shellify()`.

It's trivial to convert previously-correct code: `shellify("whatever {}", A)` => `"whatever {}"_shellify(A)`

The previous `folly::shellify()` API is still present as a transition measure. Compilers will issue a deprecation warning if it is used.

Reviewed By: yfeldblum

Differential Revision: D4435512

fbshipit-source-id: 6639cd91280dc72108e47a8a7775c5160a4e644f