Reference entry arguments are not checked for mutability.

[eddyb]

Using &mut T instead of &T for immutable-only SPIR-V storage classes (PushConstant, Uniform, etc.),
e.g. this change to examples/shaders/sky-shader/src/lib.rs compiles:

 #[spirv(fragment)]
 pub fn main_fs(
     #[spirv(frag_coord)] in_frag_coord: Vec4,
-    #[spirv(push_constant)] constants: &ShaderConstants,
+    #[spirv(push_constant)] constants: &mut ShaderConstants,
     output: &mut Vec4,
 ) {

This is likely a regression introduced by:

• Infer the storage class for images/samplers/imagesamplers #567

(an if m == expected_mutbl was accidentally removed and I missed it during review)