Process declared element segments for "possibly exported funcs" (#2851)

Now that we're using "possibly exported" as an impactful decision for
codegen (which trampolines to generate and which ABI a function has)
it's important that we calculate this property of a wasm function
correctly! Previously Wasmtime forgot to processed "declared" elements
in apart from active/passive element segments, but this updates Wasmtime
to ensure that these entries are processed and all the functions
contained within are flagged as "possibly exported".

Closes #2850

cranelift/wasm/src/environ/spec.rs

@@ -948,6 +948,13 @@ pub trait ModuleEnvironment<'data>: TargetEnvironment {
         elements: Box<[FuncIndex]>,
     ) -> WasmResult<()>;
 
+    /// Indicates that a declarative element segment was seen in the wasm
+    /// module.
+    fn declare_elements(&mut self, elements: Box<[FuncIndex]>) -> WasmResult<()> {
+        drop(elements);
+        Ok(())
+    }
+
     /// Provides the number of passive data segments up front.
     ///
     /// By default this does nothing, but implementations may use this to

cranelift/wasm/src/sections_translator.rs

@@ -400,7 +400,7 @@ pub fn parse_element_section<'data>(
                 environ.declare_passive_element(index, segments)?;
             }
             ElementKind::Declared => {
-                // Nothing to do here.
+                environ.declare_elements(segments)?;
             }
         }
     }