Implement implicit conversions on function return expressions to the function's type.

git-svn-id: https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/tools/glslang@26501 e7fa87d3-cd2b-0410-9028-fcbf551c1848

Test/420.geom

@@ -48,3 +48,8 @@ out gl_PerVertex {
     float gl_PointSize[1];  // ERROR, adding array
     float gl_ClipDistance;  // ERROR, removing array
 };
+
+float foo5()
+{
+    return i;  // implicit conversion of return type
+}