[cxx-interop] Fix a crash when calling std::any_cast from Swift

In libc++, `std::any_cast` is declared as a friend function of `std::any`.

ClangImporter was not correctly handling friend functions declared within structs within namespaces correctly.

rdar://147261941

Author: egorzhdan

test/Interop/Cxx/stdlib/Inputs/module.modulemap

@@ -1,3 +1,9 @@
+module StdAny {
+  header "std-any.h"
+  requires cplusplus
+  export *
+}
+
 module StdNumeric {
   header "std-numeric.h"
   requires cplusplus

test/Interop/Cxx/stdlib/Inputs/std-any.h

@@ -0,0 +1,6 @@
+#include <any>
+#include <string>
+
+inline std::any getStdAnyString() {
+  return std::string("abc210");
+}

test/Interop/Cxx/stdlib/use-std-any.swift

@@ -0,0 +1,23 @@
+// RUN: %target-run-simple-swift(-I %S/Inputs -cxx-interoperability-mode=upcoming-swift)
+// RUN: %target-run-simple-swift(-I %S/Inputs -cxx-interoperability-mode=upcoming-swift -Xcc -std=c++17)
+// RUN: %target-run-simple-swift(-I %S/Inputs -cxx-interoperability-mode=upcoming-swift -Xcc -std=c++20)
+
+// REQUIRES: executable_test
+
+// In Microsoft STL, all overloads of std::any_cast return a dependent templated
+// type, which Swift isn't able to instantiate. 
+// UNSUPPORTED: OS=windows-msvc
+
+import StdlibUnittest
+import StdAny
+import CxxStdlib
+
+var StdAnyTestSuite = TestSuite("StdAny")
+
+StdAnyTestSuite.test("std.any_cast<std.string>") {
+  let a1 = getStdAnyString()
+  let c1 = std.any_cast(a1) as std.string
+  expectEqual(c1, std.string("abc210"))
+}
+
+runAllTests()