Although I don't plan to bring over new assertions wholesale
into the current qualification branch, it's entirely possible
that various minor changes in main will use the new assertions;
having this basic support in the release branch will simplify that.
(This is why I'm adding the includes as a separate pass from
rewriting the individual assertions)
Start `linear_function` canonicalization skeleton copying from
`differentiable_function` canonicalization. For now, transpose function
operands are filled in with `undef`.
Clarify `llvm_unreachable` after exhaustive switch is needed to silence
MSVC C4715, so we don't accidentally remove it.
Standardize some assertion messages.
Move differentiation-related SILOptimizer files to
{include/swift,lib}/SILOptimizer/Differentiation/.
This reduces directory nesting and gathers files together.
