expose value of constant integers in module reflection

[ennis]

One of the pain points of working with shaders is needing to keep the definitions of interface types in sync between shader and device code. With the reflection API of slang, it is possible to automate this process by generating a header file for the host language from the reflection data, that contains all interface types used in the shader.
However, one thing missing is the ability to get the value of compile-time constants defined in the shaders. This is useful if you have some constant that an algorithm or a data structure depends on, like a static block or tile size, and you don't want to have to keep them in sync manually between host and shader.

To solve this (at least for integers) this PR adds VariableReflection::getDefaultValueInt to get the default value of a variable if it is a compile-time constant integer. The reflection-api example has also been updated to print all the static const variables of a module and their values.

Internally, this uses the VarDeclBase::val field that should contain the constant-folded initializer value. However, currently this field is null if, for instance, the initializer is a variable reference (as is the case for THREADGROUP_SIZE_Y in the compute-simple.slang of the reflection-api example). It seems to be initialized in SemanticsDeclHeaderVisitor::checkVarDeclCommon, which then goes through SemanticsVisitor::tryConstantFoldExpr, which does try to follow variable references. However it seems that at this point the reference cannot be resolved. I'd like to make this work before trying to merge this.

[CLAassistant]

All committers have signed the CLA.

[ennis]

Following a suggestion on discord I've added code to update VarDecl's constant value at a later stage during DeclBodyVisitor. Since constant folding is already done inside _validateCircularVarDefinition, this commit reuses the constant that is evaluated there.

[csyonghe]

Looks good to me!