[cxx-interop] Fix test header to declare linkage for a implicit template (#72522)

The header was defining a function, the function created a lambda, and
the lambda was transformed into a `std::function`. This transformation
is incorrect because the function scope does not have linkage, so the
instantiated types will not have linkage either, causing the error
below.

```
.../include/c++/11.2.0/bits/invoke.h:104:5:
error: function 'std::__invoke_r<int, (lambda at .../swift/test/Interop/Cxx/stdlib/Inputs/std-function.h:9:10) &, int>' is used but not defined in this translation unit, and cannot be defined in any other translation unit because its type does not have linkage
102 │   template<typename _Res, typename _Callable, typename... _Args>
103 │     constexpr enable_if_t<is_invocable_r_v<_Res, _Callable,
    _Args...>, _Res>
    104 │     __invoke_r(_Callable&& __fn, _Args&&... __args)
        │     ╰─ error: function 'std::__invoke_r<int, (lambda at .../swift/test/Interop/Cxx/stdlib/Inputs/std-function.h:9:10) &, int>' is used but not defined in this translation unit, and cannot be defined in any other translation unit because its type does not have linkage
        105 │     noexcept(is_nothrow_invocable_r_v<_Res, _Callable,
            _Args...>)
  106 │     {

  }
```

Declaring the function `inline` forces each TU to have their own copies
of the function, which avoids the instantiated templates from being in a
different TU than the one using them.

The header would not have worked in a normal C++ program, since none of
the functions declare linkage, they would have been defined in each TU
that included the header and it would fail linking as soon as two of
those TU tried to be linked together. Declaring the functions `inline`
avoids the problem (a more normal header, only with declarations, would
also worked).

Author: drodriguez

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

@@ -5,12 +5,12 @@
 
 using FunctionIntToInt = std::function<int(int)>;
 
-FunctionIntToInt getIdentityFunction() {
+inline FunctionIntToInt getIdentityFunction() {
   return [](int x) { return x; };
 }
 
-bool isEmptyFunction(FunctionIntToInt f) { return !(bool)f; }
+inline bool isEmptyFunction(FunctionIntToInt f) { return !(bool)f; }
 
-int invokeFunction(FunctionIntToInt f, int x) { return f(x); }
+inline int invokeFunction(FunctionIntToInt f, int x) { return f(x); }
 
 #endif // TEST_INTEROP_CXX_STDLIB_INPUTS_STD_FUNCTION_H