Add ability to specialize generic references to functions, types and more (#4909)

* More reflection API features.

+ Lookup methods and members (by string) on types
+ Fix issue with looking up non-static members through the scope operator '::'
+ `GenericReflection`: Cast a decl to generic to access unspecialized generic parameter names and constraints
+ `GenericReflection`: Use `getGenericContainer()` from function, variable or type to access the 'nearest' generic parent along with specialization info
+ `GenericReflection::getConcreteType` and `GenericReflection::getConcreteIntVal`: to get the concrete type of a param in the context of the reflection object
+ `GenericReflection::getOuterGenericContainer` to go up one level and get the outer generic declarations (if there are more than one enclosing generic scopes)
+ `DeclReflection::getParent`: go to parent declaration.
+ Change `VariableReflection` to be a `DeclRef` rather than a decl (allows us to return properly substituted types for methods, members, and more)

* Fix Falcor issue

* Initial namespace reflection support

* FIx issue with specializing witness tables

* Add API method for specializing parameters of a generic decl

* Add ability to specialize generic references to functions, types and more

This PR adds the following end-points:

- `specializeGeneric()` method that can be called on a generic reflection to substitute arguments for generic type and value parameters. It returns another generic reflection, but this time with the appropriate substitution.
- `applySpecializations()` method to then copy these specializations onto an existing type or function reflection.
- `isSubType()` to check if a type is a subtype of another type (useful to check if a type is differentiable by checking `IDifferentiable`)

This PR also:
 - Adds `DeclReflection::Kind::Namespace` so that namespace containers are correctly reflected when walking the decl-tree. the name can be obtained through `getName()` but there's no need to cast to a namespace (since there's nothing else we can do with a namespace decl)
 - Fixes an issue with name-based lookups that fail if a type or function is referenced without specializations. Its helpful to be able to form a reference to a function with default substitutions, so that we can we can specialize it later (either directly, or via argument types).

* Update slang.h

* Fix up naming

* Update slang-compiler.h

* Update slang-reflection-api.cpp

* Update slang.cpp

* Update slang.cpp

* Update slang.cpp

* Use `checkGenericAppWithCheckedArgs` to do specialization

* Update slang-reflection-api.cpp

* Update slang-check-decl.cpp

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