Support scalarization of varying input/output for GLSL

GLSL technically supports varying (`in`, `out`) parameters of `struct` type, but there are some annoying constraints (not allowed for VS input), and it doesn't work with how an HLSL user would usually put "system-value" inputs/outputs into a `struct` together with ordinary inputs/outputs.

To work around this, this change adds support for using an imported Slang `struct` type for an `in` or `out` parameter, in which case it will (1) be scalarized and (2) will have system-value semantics mapped appropriately, just as for an entry-point parameter when cross-compiling an HLSL-style `main()`.

Changes:

- Add a notion of a `VaryingTupleExpr` and `VaryingTupleVarDecl`, similar to those for the resources-in-structs case

- Trigger use of these when we have a global-scope varying in/out using an imported `struct` type

- Also use these in the cross-compilation case for ordinary varying input/output (since this approach seems like it should be more general, and can hopefully handle stuff like GS input/output some day)

- When generating parameter binding information, special case global-scope input/output, and treat it the same as entry-point-parameter input/output

- Revamp how used resource ranges are computed so that we can eventually make this specific to an entry point

- Actually implement first signs of life for `maybeMoveTemp` so that assignments to the tuple-ified outputs will work better

- Add first test case that actually seems to work

- Add diagnostics for conflicting explicit bindings on a parameter

- Add diagnostic for different parameters with overlapping bindings

- Make global-scope varying input/output use a tracking data structure specific to the translation unit for computing locations (so that they are independent of other TUs)

1 files changed, 6 insertions, 0 deletions

diff --git a/source/slang/check.cpp b/source/slang/check.cpp
index b175b7f86..1c1c6d5d3 100644
--- a/source/slang/check.cpp
+++ b/source/slang/check.cpp
@@ -4801,6 +4801,12 @@ namespace Slang
             return expr;
         }
 
+        RefPtr<ExpressionSyntaxNode> visitAggTypeCtorExpr(AggTypeCtorExpr* expr)
+        {
+            assert(!"unexpected");
+            return expr;
+        }
+
         //
         //
         //