[llvm-branch-commits] [libcxx] change the visibility of libc++ header to public in libcxx module (PR #91182)

[via llvm-branch-commits]

https://github.com/RichardLuo0 created https://github.com/llvm/llvm-project/pull/91182

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

int main(int, const char **) { }
```
boost will include something from libc++, but we are using [-nostdinc++](https://github.com/RichardLuo0/llvm-project/blob/15fdd47c4b110b64dc61f636e42e0484bf8bdbe0/libcxx/modules/CMakeLists.txt.in#L52), so the compiler can not find any default std header.
Therefore the locally built header needs to be public.

>From 15fdd47c4b110b64dc61f636e42e0484bf8bdbe0 Mon Sep 17 00:00:00 2001
From: RichardLuo <22096448+RichardLuo0 at users.noreply.github.com>
Date: Mon, 6 May 2024 18:16:24 +0800
Subject: [PATCH] change the visibility of libc++ header to public

---
 libcxx/modules/CMakeLists.txt.in | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libcxx/modules/CMakeLists.txt.in b/libcxx/modules/CMakeLists.txt.in
index e332d70cc16333..1f8f159e710cb9 100644
--- a/libcxx/modules/CMakeLists.txt.in
+++ b/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)
@@ -67,7 +67,7 @@ target_sources(std.compat
     std.compat.cppm
 )
 
-target_include_directories(std.compat SYSTEM PRIVATE @LIBCXX_CONFIGURED_INCLUDE_DIRS@)
+target_include_directories(std.compat SYSTEM PUBLIC @LIBCXX_CONFIGURED_INCLUDE_DIRS@)
 
 if (NOT @LIBCXX_ENABLE_EXCEPTIONS@)
   target_compile_options(std.compat PUBLIC -fno-exceptions)