[clang] NFC: Make clang resource headers an interface library (PR #88317)

[Evan Wilde via cfe-commits]

https://github.com/etcwilde created https://github.com/llvm/llvm-project/pull/88317

Making the clang resource headers into an interface library instead of a custom target means that we can attach the header search paths to the library. Targets that "link" against this library will automatically have the appropriate paths added to their header search paths to find them. Then downstream projects that embed a copy of clang can query the generated `ClangTargets.cmake` file for the header location instead of attempting to compute them.

>From de73a407bdf589d98c1c5a250100318fa2dfbe45 Mon Sep 17 00:00:00 2001
From: Evan Wilde <etceterawilde at gmail.com>
Date: Thu, 14 Mar 2024 18:11:24 -0700
Subject: [PATCH] Make clang resource headers an interface library

Making the clang resource headers into an interface library means that
we can attach the header search paths to the library. Targets that
"link" against this library will automatically have the appropriate
paths added to their header search paths to find them.
---
 clang/lib/Headers/CMakeLists.txt | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Headers/CMakeLists.txt b/clang/lib/Headers/CMakeLists.txt
index 97104ccd8db59c..0367f611526f41 100644
--- a/clang/lib/Headers/CMakeLists.txt
+++ b/clang/lib/Headers/CMakeLists.txt
@@ -437,14 +437,14 @@ foreach( f ${generated_files} )
 endforeach( f )
 
 function(add_header_target target_name file_list)
-  add_custom_target(${target_name} DEPENDS ${file_list})
+  add_library(${target_name} INTERFACE ${file_list})
   set_target_properties(${target_name} PROPERTIES
     FOLDER "Misc"
     RUNTIME_OUTPUT_DIRECTORY "${output_dir}")
 endfunction()
 
 # The catch-all clang-resource-headers target
-add_custom_target("clang-resource-headers" ALL DEPENDS ${out_files})
+add_library(clang-resource-headers INTERFACE ${out_files})
 set_target_properties("clang-resource-headers" PROPERTIES
   FOLDER "Misc"
   RUNTIME_OUTPUT_DIRECTORY "${output_dir}")
@@ -501,6 +501,10 @@ add_header_target("windows-resource-headers" ${windows_only_files})
 add_header_target("utility-resource-headers" ${utility_files})
 
 get_clang_resource_dir(header_install_dir SUBDIR include)
+target_include_directories(clang-resource-headers INTERFACE
+  $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
+  $<INSTALL_INTERFACE:${header_install_dir}>)
+set_property(GLOBAL APPEND PROPERTY CLANG_EXPORTS clang-resource-headers)
 
 #############################################################
 # Install rules for the catch-all clang-resource-headers target