[cxx-interop] Add a test for interface generation of a module requiring C++20

rdar://148840350

Author: egorzhdan

test/SourceKit/InterfaceGen/Inputs/module.modulemap

@@ -0,0 +1,4 @@
+module UsesCXX20 {
+  header "uses-cxx20.h"
+  export *
+}

test/SourceKit/InterfaceGen/Inputs/uses-cxx20.h

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+#include <span>
+
+using MySpanOfInt = std::span<int>;

test/SourceKit/InterfaceGen/gen_clang_cxx_module_with_cxx20.swift

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+// First, check that we error out for older C++ standards.
+// RUN: not %sourcekitd-test -req=interface-gen -module UsesCXX20 -- -cxx-interoperability-mode=default -Xfrontend -disable-implicit-concurrency-module-import -Xfrontend -disable-implicit-string-processing-module-import -I %S/Inputs -target %target-triple -sdk %sdk
+
+// Now make sure we emit the correct interface for std::span if C++20 is enabled.
+// RUN: %sourcekitd-test -req=interface-gen -module UsesCXX20 -- -cxx-interoperability-mode=default -Xfrontend -disable-implicit-concurrency-module-import -Xfrontend -disable-implicit-string-processing-module-import -I %S/Inputs -target %target-triple -sdk %sdk -Xcc -std=c++20 | %FileCheck %s
+
+// FIXME: older libstdc++ version on Swift CI does not support C++20
+// UNSUPPORTED: OS=linux-gnu
+
+// CHECK: public typealias MySpanOfInt = std.span<