Handle when type alias's type parameter extends type that wont get emitted in .d.ts

Fixes #12326

Author: sheetalkamat

src/compiler/declarationEmitter.ts

@@ -1037,6 +1037,10 @@ namespace ts {
                             diagnosticMessage = Diagnostics.Type_parameter_0_of_exported_function_has_or_is_using_private_name_1;
                             break;
 
+                        case SyntaxKind.TypeAliasDeclaration:
+                            diagnosticMessage = Diagnostics.Type_parameter_0_of_exported_type_alias_has_or_is_using_private_name_1;
+                            break;
+
                         default:
                             Debug.fail("This is unknown parent for type parameter: " + node.parent.kind);
                     }

src/compiler/diagnosticMessages.json

@@ -2276,6 +2276,10 @@
         "category": "Error",
         "code": 4082
     },
+    "Type parameter '{0}' of exported type alias has or is using private name '{1}'.": {
+        "category": "Error",
+        "code": 4083
+    },
     "Conflicting definitions for '{0}' found at '{1}' and '{2}'. Consider installing a specific version of this library to resolve the conflict.": {
         "category": "Message",
         "code": 4090

tests/baselines/reference/declarationEmitTypeAliasTypeParameterExtendingUnknownSymbol.errors.txt

@@ -0,0 +1,11 @@
+tests/cases/compiler/declarationEmitTypeAliasTypeParameterExtendingUnknownSymbol.ts(2,18): error TS2304: Cannot find name 'Unknown'.
+tests/cases/compiler/declarationEmitTypeAliasTypeParameterExtendingUnknownSymbol.ts(2,18): error TS4083: Type parameter 'T' of exported type alias has or is using private name 'Unknown'.
+
+
+==== tests/cases/compiler/declarationEmitTypeAliasTypeParameterExtendingUnknownSymbol.ts (2 errors) ====
+    
+    type A<T extends Unknown> = {}
+                     ~~~~~~~
+!!! error TS2304: Cannot find name 'Unknown'.
+                     ~~~~~~~
+!!! error TS4083: Type parameter 'T' of exported type alias has or is using private name 'Unknown'.

tests/baselines/reference/declarationEmitTypeAliasTypeParameterExtendingUnknownSymbol.js

@@ -0,0 +1,5 @@
+//// [declarationEmitTypeAliasTypeParameterExtendingUnknownSymbol.ts]
+
+type A<T extends Unknown> = {}
+
+//// [declarationEmitTypeAliasTypeParameterExtendingUnknownSymbol.js]

tests/cases/compiler/declarationEmitTypeAliasTypeParameterExtendingUnknownSymbol.ts

@@ -0,0 +1,3 @@
+// @declaration: true
+
+type A<T extends Unknown> = {}