Refactor substitution representation in DeclRefBase (#363)

This commit changes the type of `DeclRefBase::substitutions` from `RefPtr<Substitutions>` to `SubstitutionSet`, which is a new type defined as following:
```
struct SubstitutionSet
{
    RefPtr<GenericSubstitution> genericSubstitutions;
    RefPtr<ThisTypeSubstitution> thisTypeSubstitution;
    RefPtr<GlobalGenericParamSubstitution> globalGenParamSubstitutions;
}
```
This change get rid of most helper functions to retreive the substitution of a certain type, as well as surgery operations to insert a `ThisTypeSubstitution` or `GlobalGenericTypeSubstittuion` at top or bottom of the substitution chain. It also simplies type comparison when certain type of substitution should not be considered as part of type definition.

1 files changed, 1 insertions, 2 deletions

diff --git a/source/slang/legalize-types.cpp b/source/slang/legalize-types.cpp
index 1264201a2..211685aa2 100644
--- a/source/slang/legalize-types.cpp
+++ b/source/slang/legalize-types.cpp
@@ -450,8 +450,7 @@ static RefPtr<Type> createBuiltinGenericType(
     // TODO: we should have library code to make
     // manipulations like this way easier.
 
-    RefPtr<GenericSubstitution> oldGenericSubst = getGenericSubstitution(
-        typeDeclRef.substitutions);
+    RefPtr<GenericSubstitution> oldGenericSubst = typeDeclRef.substitutions.genericSubstitutions;
     SLANG_ASSERT(oldGenericSubst);
 
     RefPtr<GenericSubstitution> newGenericSubst = new GenericSubstitution();