[libc++] change the visibility of libc++ header to public in libcxx module (#91240)

This PR addresses a problem that headers may not be able to be found if
`#include` is used with std modules.

Consider the following file:

    #include <boost/json.hpp>
    import std;

    int main(int, const char **) { }


Boost will include something from libc++, but we are using -nostdinc++
at [1] so the compiler can not find any default std header. Therefore 
the locally built header needs to be public.

[1]: https://github.com/RichardLuo0/llvm-project/blob/15fdd47c4b110b64dc61f636e42e0484bf8bdbe0/libcxx/modules/CMakeLists.txt.in#L52

Author: RichardLuo0

libcxx/modules/CMakeLists.txt.in

@@ -41,7 +41,7 @@ target_sources(std
     std.cppm
 )
 
-target_include_directories(std SYSTEM PRIVATE @LIBCXX_CONFIGURED_INCLUDE_DIRS@)
+target_include_directories(std SYSTEM PUBLIC @LIBCXX_CONFIGURED_INCLUDE_DIRS@)
 
 if (NOT @LIBCXX_ENABLE_EXCEPTIONS@)
   target_compile_options(std PUBLIC -fno-exceptions)