Add parsing for function pointer types (#39879)

Add parsing for function pointer types

Author: 333fred

src/Compilers/CSharp/Portable/CSharpResources.Designer.cs

@@ -5945,6 +5945,12 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_BadSwitchValue" xml:space="preserve">
     <value>Command-line syntax error: '{0}' is not a valid value for the '{1}' option. The value must be of the form '{2}'.</value>
   </data>
+  <data name="IDS_FeatureFunctionPointers" xml:space="preserve">
+    <value>function pointers</value>
+  </data>
+  <data name="ERR_InvalidFunctionPointerCallingConvention">
+    <value>'{0}' is not a valid calling convention for a function pointer. Valid conventions are 'cdecl', 'managed', 'unmanaged', 'thiscall', and 'stdcall'.</value>
+  </data>
   <data name="ERR_InternalError" xml:space="preserve">
     <value>Internal error in the C# compiler.</value>
   </data>

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -1736,6 +1736,12 @@ internal enum ErrorCode
 
         ERR_InternalError = 8751,
 
+        #region diagnostics introduced in preview
+
+        ERR_InvalidFunctionPointerCallingConvention = 8752,
+
+        #endregion
+
         // Note: you will need to re-generate compiler code after adding warnings (eng\generate-compiler-code.cmd)
     }
 }

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

@@ -185,6 +185,7 @@ internal enum MessageID
         IDS_FeatureSwitchExpression = MessageBase + 12763,
         IDS_FeatureAsyncUsing = MessageBase + 12764,
         IDS_FeatureLambdaDiscardParameters = MessageBase + 12765,
+        IDS_FeatureFunctionPointers = MessageBase + 12766,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -293,6 +294,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
             {
                 // Preview features.
                 case MessageID.IDS_FeatureLambdaDiscardParameters: // semantic check
+                case MessageID.IDS_FeatureFunctionPointers:
                     return LanguageVersion.Preview;
 
                 // C# 8.0 features.