MSL: Fix regression with Private parameter declaration.

If we compile multiple times due to forced_recompile, we had
deferred_declaration = true while emitting function prototypes which
broke an assumption. Fix this by clearing out stale state before leaving
a function.

reference/opt/shaders-msl/frag/private-variable-prototype-declaration.frag

@@ -0,0 +1,17 @@
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+struct main0_out
+{
+    float3 FragColor [[color(0)]];
+};
+
+fragment main0_out main0()
+{
+    main0_out out = {};
+    out.FragColor = float3(1.0);
+    return out;
+}
+

reference/shaders-msl/frag/private-variable-prototype-declaration.frag

@@ -0,0 +1,39 @@
+#pragma clang diagnostic ignored "-Wmissing-prototypes"
+
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+struct AStruct
+{
+    float4 foobar;
+};
+
+struct main0_out
+{
+    float3 FragColor [[color(0)]];
+};
+
+void someFunction(thread AStruct& s)
+{
+    s.foobar = float4(1.0);
+}
+
+void otherFunction(thread float3& global_variable)
+{
+    global_variable = float3(1.0);
+}
+
+fragment main0_out main0()
+{
+    main0_out out = {};
+    AStruct param;
+    someFunction(param);
+    AStruct inputs = param;
+    float3 global_variable;
+    otherFunction(global_variable);
+    out.FragColor = global_variable;
+    return out;
+}
+

shaders-msl/frag/private-variable-prototype-declaration.frag

@@ -0,0 +1,20 @@
+#version 450
+
+struct AStruct { vec4 foobar; };
+
+void someFunction(out AStruct s) { s.foobar = vec4(1.0); }
+
+highp vec3 global_variable;
+
+void otherFunction() {
+    global_variable = vec3(1.0);
+}
+
+layout(location = 0) out vec3 FragColor;
+
+void main() {
+	AStruct inputs;
+	someFunction(inputs);
+    otherFunction();
+	FragColor = global_variable;
+}

spirv_glsl.cpp

@@ -10660,6 +10660,14 @@ void CompilerGLSL::emit_function(SPIRFunction &func, const Bitset &return_flags)
 	end_scope();
 	processing_entry_point = false;
 	statement("");
+
+	// Make sure deferred declaration state for local variables is cleared when we are done with function.
+	// We risk declaring Private/Workgroup variables in places we are not supposed to otherwise.
+	for (auto &v : func.local_variables)
+	{
+		auto &var = get<SPIRVariable>(v);
+		var.deferred_declaration = false;
+	}
 }
 
 void CompilerGLSL::emit_fixup()