Fix up scoping for cross-compiled `main()` body
Fix up scoping for cross-compiled `main()` body

The earier changes to add sequence statements and change how the `isBuildingStmt` logic in lowering works doesn't work for this logic, which assumes it can just set `isBuildingStmt` and be sure that decls will go into the right place.

shader-slang/slang,shader-slang/slang,csyonghe/slang,csyonghe/slang,shader-slang/slang,csyonghe/slang