VideoCommon: Remove unnecessary memset on ShaderUid instances.

Zero-initialization zeroes out all members and padding bits, so this is
safe to do. While we're at it, also add static assertions that enforce
the necessary requirements of a UID type explicitly within the ShaderUid
class.

This way, we can remove several memset calls around the shader
generation code that makes sure the underlying UID data is zeroed out.
Now our ShaderUid class enforces this for us, so we don't need to care about
it at the usage sites.

Source/Core/VideoCommon/UberShaderPixel.h

@@ -21,7 +21,7 @@ struct pixel_ubershader_uid_data
 };
 #pragma pack()
 
-typedef ShaderUid<pixel_ubershader_uid_data> PixelShaderUid;
+using PixelShaderUid = ShaderUid<pixel_ubershader_uid_data>;
 
 PixelShaderUid GetPixelShaderUid();