Add a utility for principal types (#7642)

WebAssembly has the property that every valid sequence of instructions
has a principal type, i.e. a unique most-specific type. However, due to
various kinds of polymorphism, these principal types may contain type
variables. Specifically, the syntaxes for value types, heap types,
nullability, sharedness, and exactness are all augmented with type
variables.

Add a PrincipalType utility for representing and composing these
principal types. This can be seen as a more powerful version of the
existing StackSignature utility. Follow-up PRs will update ChildTyper to
use principal types to represent type constraints and use the principal
types of outlined instruction sequences to detect polymorphism and
determine the types of outlined functions. There are also further
potential applications in simplifying validation, simplifying expression
typing, and performing spec-compliant validation.

Author: tlively

src/ir/CMakeLists.txt

@@ -16,6 +16,7 @@ set(ir_SOURCES
   properties.cpp
   LocalGraph.cpp
   LocalStructuralDominance.cpp
+  principal-type.cpp
   public-type-validator.cpp
   ReFinalize.cpp
   return-utils.cpp