Add facet type values and an instruction that produces them (#4460)

Still to do:
* Represent facet type values in a canonical form
* Produce & consume facet type values instead of interface values
* `type` should be associated with a canonical facet type value
* Support `&` on facet type values
* Type check and enforce requirements in facet types

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Co-authored-by: Josh L <josh11b@users.noreply.github.com>

toolchain/sem_ir/file.cpp

@@ -256,6 +256,7 @@ auto GetExprCategory(const File& file, InstId inst_id) -> ExprCategory {
       case ClassType::Kind:
       case CompleteTypeWitness::Kind:
       case ConstType::Kind:
+      case FacetType::Kind:
       case FacetTypeAccess::Kind:
       case FloatLiteral::Kind:
       case FloatType::Kind: