Exempt ambient [#]private from unused error (microsoft#37050)

* Exempt ambient [#]private from unused error

These declarations exist to create nominality so they _must_ be unused.
There should be no error for them.

* Switch to fourslash test

I don't know how to baseline suggestion diagnostics in the compiler
tests.

Author: sandersn

src/compiler/checker.ts

@@ -30566,7 +30566,9 @@ namespace ts {
                             break;
                         }
                         const symbol = getSymbolOfNode(member);
-                        if (!symbol.isReferenced && (hasModifier(member, ModifierFlags.Private) || isNamedDeclaration(member) && isPrivateIdentifier(member.name))) {
+                        if (!symbol.isReferenced
+                            && (hasModifier(member, ModifierFlags.Private) || isNamedDeclaration(member) && isPrivateIdentifier(member.name))
+                            && !(member.flags & NodeFlags.Ambient)) {
                             addDiagnostic(member, UnusedKind.Local, createDiagnosticForNode(member.name!, Diagnostics._0_is_declared_but_its_value_is_never_read, symbolToString(symbol)));
                         }
                         break;

tests/cases/fourslash/unusedVariableInClass4.ts

@@ -0,0 +1,11 @@
+/// <reference path='fourslash.ts' />
+
+// @noUnusedLocals: false
+////class greeter {
+////    [|private greeting: string;|]
+////}
+
+verify.codeFix({
+    description: "Remove unused declaration for: 'greeting'",
+    newRangeContent: "",
+});

tests/cases/fourslash/unusedVariableInClass5.ts

@@ -0,0 +1,10 @@
+/// <reference path='fourslash.ts' />
+
+// @noUnusedLocals: true
+// @target: esnext
+////declare class greeter {
+////    #private;
+////    private name;
+////}
+
+verify.noErrors()