[clang] [HLSL][SPIRV] Add vk::constant_id attribute. (PR #143544)

[Nathan Gauër via cfe-commits]

================
@@ -12919,6 +12919,18 @@ def err_spirv_enum_not_int : Error<
 def err_spirv_enum_not_valid : Error<
    "invalid value for %select{storage class}0 argument">;
 
+def err_specialization_const_lit_init
+    : Error<"variable with 'vk::constant_id' attribute cannot have an "
+            "initializer that is not a literal">;
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Keenuts wrote:

nit, more aligned with C++ errors about const init.

```suggestion
def err_specialization_const_lit_init
    : Error<"variable with 'vk::constant_id' attribute must be initialized by a int/float/enum/bool literal.">;
```

nit nit: maybe this message could also be used to replace the 2 other messages: this way you have a full fix suggested instead of first "this is not initialized", then "now it's initialized but not a literal", and finally "this is a literal, but not a float/int/enum/bool".

https://github.com/llvm/llvm-project/pull/143544