Type annotations for instruction operands are omitted, e.g.
```
%3 = struct $S(%1, %2)
```
Operand types are redundant anyway and were only used for sanity checking in the SIL parser.
But: operand types _are_ printed if the definition of the operand value was not printed yet.
This happens:
* if the block with the definition appears after the block where the operand's instruction is located
* if a block or instruction is printed in isolation, e.g. in a debugger
The old behavior can be restored with `-Xllvm -sil-print-types`.
This option is added to many existing test files which check for operand types in their check-lines.
There aren't sufficiently strong compiler checks that would prevent unavailable
elements of clang enums from being instantiated at runtime, so don't consider
any clang enum elements as unavailable during lowering. Since there aren't any
symbols associated with clang enum elements, we don't have to worry about
linker failures that might result from leaving references to them in SIL.
