v0.0.0-6507-g245ca1f62

[xls][mlir] Add `extern_sproc` and `extern_eproc` ops

These ops allow to reference procedures external to the current module. Support has
been added in -elaborate-procs to elaborate from extern_sproc to extern_eproc.

No support has been added for linking modules and "resolving" the externs.

PiperOrigin-RevId: 703196503