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, 1 deletions

diff --git a/source/slang/mangle.h b/source/slang/mangle.h
index 406796951..45b1d4bd1 100644
--- a/source/slang/mangle.h
+++ b/source/slang/mangle.h
@@ -12,7 +12,7 @@ namespace Slang
     String getMangledName(DeclRef<Decl> const & declRef);
     String getMangledName(DeclRefBase const & declRef);
 
-    String mangleSpecializedFuncName(String baseName, RefPtr<Substitutions> subst);
+    String mangleSpecializedFuncName(String baseName, SubstitutionSet subst);
     String getMangledNameForConformanceWitness(
         Type* sub,
         Type* sup);