This is used for synthetic uses like _ = x that do not act as a true use but
instead only suppress unused variable warnings. This patch just adds the
instruction.
Eventually, we can use it to move the unused variable warning from Sema to SIL
slimmming the type checker down a little bit... but for now I am using it so
that other diagnostic passes can have a SIL instruction (with SIL location) so
that we can emit diagnostics on code like _ = x. Today we just do not emit
anything at all for that case so a diagnostic SIL pass would not see any
instruction that it could emit a diagnostic upon. In the next patch of this
series, I am going to add SILGen support to do that.
With a focus on updating the documentation of Ownership SSA.
The main changes are:
* Created a new directory `docs/SIL` and moved all SIL-related files into this directory.
* Converted `rst` files to markdown.
* Extracted sections from `SIL.md` which go into very much detail - including the instruction reference - into separate files: `Types.md`, `Ownership.md`, `FunctionAttributes.md`, `Instructions.md`. Those files are referenced from `SIL.md` at the relevant places.
* Rewrote and updated the OSSA part in `SIL.md`
* Removed a few sections, which are not relevant anymore, like "Value dependency" (which is replaced by ownership concepts).
* Fixed and improved a lot of small things.
