Validate diagnostic ID in Experimental attribute

Author: Julien Couvreur

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -7809,4 +7809,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_ExpectedInterpolatedString" xml:space="preserve">
     <value>Expected interpolated string</value>
   </data>
+  <data name="ERR_InvalidExperimentalDiagID" xml:space="preserve">
+    <value>The argument to the 'Experimental' attribute must be a valid identifier</value>
+  </data>
 </root>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -2275,6 +2275,8 @@ internal enum ErrorCode
         ERR_InterceptorGlobalNamespace = 9206,
         ERR_InterceptableMethodMustBeOrdinary = 9207,
 
+        ERR_InvalidExperimentalDiagID = 9208,
+
         #endregion
 
         // Note: you will need to do the following after adding warnings:

src/Compilers/CSharp/Portable/Symbols/Symbol_Attributes.cs

@@ -226,6 +226,13 @@ protected void DecodeWellKnownAttribute(ref DecodeWellKnownAttributeArguments<At
                 return;
             }
 
+            if (arguments.Attribute.IsTargetAttribute(this, AttributeDescription.ExperimentalAttribute)
+                && !SyntaxFacts.IsValidIdentifier((string?)arguments.Attribute.CommonConstructorArguments[0].ValueInternal))
+            {
+                arguments.Diagnostics.DiagnosticBag.Add(ErrorCode.ERR_InvalidExperimentalDiagID, arguments.AttributeSyntaxOpt.Location);
+                return;
+            }
+
             DecodeWellKnownAttributeImpl(ref arguments);
         }
 #nullable disable