Reland "[gn build] port 48e4b0f (__config_site)"

This reverts commit 13aff21f0d,
since the CMake part relanded in c06a8f9caa.

The GN part is a bit simpler than last time due to the
prior simplifications in acea470c16.

llvm/utils/gn/secondary/clang/tools/driver/BUILD.gn

@@ -86,7 +86,7 @@ executable("clang") {
     # clang.  This is different from the CMake build, which requires devs to
     # explicitly build the "libcxx" target (which also needlessly compiles the
     # libcxx sources) to get a working compiler.
-    deps += [ "//libcxx/include" ]
+    deps += [ "//libcxx/include:copy_headers" ]
   }
   sources = [
     "cc1_main.cpp",

llvm/utils/gn/secondary/libcxx/include/BUILD.gn

@@ -1,9 +1,19 @@
 import("//libcxx/config.gni")
 import("//llvm/utils/gn/build/write_cmake_config.gni")
 
-write_cmake_config("write_config") {
+libcxx_generated_include_dir = "$root_build_dir/include/c++/v1"
+
+# This is a bit weird. For now, we assume that __config_site is identical
+# in all toolchains, and only copy it (and all other libcxx headers)
+# to 'include' in the root build dir, so that it's the same for all toolchains.
+# Maybe we wnt to make this per-toolchain eventually (and then use root_out_dir
+# in libcxx_generated_include_dir) -- e.g. for cross-builds that for example
+# use for-linux-configured libc++ for the host build but for-windows-configured
+# libc++ for the target build.
+if (current_toolchain == default_toolchain) {
+  write_cmake_config("write_config_site") {
     input = "__config_site.in"
-  output = "$target_gen_dir/__config_site"
+    output = "$libcxx_generated_include_dir/__config_site"
 
     values = [
       "_LIBCPP_ABI_FORCE_ITANIUM=",
@@ -47,25 +57,7 @@ write_cmake_config("write_config") {
     }
   }
 
-# Generate a custom __config header. The new header is created
-# by prepending __config_site to the current __config header.
-action("concat_config") {
-  script = "//libcxx/utils/cat_files.py"
-  inputs = [
-    "$target_gen_dir/__config_site",
-    "__config",
-  ]
-  outputs = [ "$root_build_dir/include/c++/v1/__config" ]
-  args = [
-    rebase_path("$target_gen_dir/__config_site", root_build_dir),
-    rebase_path("__config", root_build_dir),
-    "-o",
-    rebase_path(outputs[0], root_build_dir),
-  ]
-  deps = [ ":write_config" ]
-}
-
-copy("include") {
+  copy("copy_headers") {
     sources = [
       "__availability",
       "__bit_reference",
@@ -257,7 +249,7 @@ copy("include") {
       "wchar.h",
       "wctype.h",
     ]
-  deps = [ ":concat_config" ]
+    deps = [ ":write_config_site" ]
     if (target_os != "mac" && target_os != "win") {
       # libcxx/cmake/Modules/HandleLibCXXABI.cmake sets
       # LIBCXX_CXX_ABI_HEADER_TARGET if the libcxx abi library either of
@@ -271,3 +263,16 @@ copy("include") {
     }
     outputs = [ "$root_build_dir/include/c++/v1/{{source_target_relative}}" ]
   }
+}
+
+config("include_config") {
+  include_dirs = [ libcxx_generated_include_dir ]
+}
+
+
+group("include") {
+  deps = [
+    ":copy_headers($default_toolchain)",
+  ]
+  public_configs = [ ":include_config" ]
+}

llvm/utils/gn/secondary/libcxx/src/BUILD.gn

@@ -1,4 +1,5 @@
 import("//clang/runtimes.gni")
+import("//libcxx/config.gni")
 import("//llvm/utils/gn/build/symlink_or_copy.gni")
 
 declare_args() {
@@ -37,10 +38,7 @@ declare_args() {
 }
 
 config("cxx_config") {
-  include_dirs = [
-    "//libcxxabi/include",
-    "//libcxx/include",
-  ]
+  include_dirs = [ "//libcxxabi/include" ]
   cflags = [
     "-Wall",
     "-Wextra",
@@ -212,6 +210,7 @@ if (libcxx_enable_shared) {
     sources = cxx_sources
     deps = [
       "//compiler-rt/lib/builtins",
+      "//libcxx/include",
       "//libcxxabi/src:cxxabi_shared",
       "//libunwind/src:unwind_shared",
     ]
@@ -261,6 +260,7 @@ if (libcxx_enable_static) {
     }
     deps = [
       "//compiler-rt/lib/builtins",
+      "//libcxx/include",
       "//libcxxabi/src:cxxabi_static",
       "//libunwind/src:unwind_static",
     ]
@@ -277,6 +277,7 @@ if (libcxx_enable_experimental) {
     output_dir = runtimes_dir
     output_name = "c++experimental"
     sources = [ "experimental/memory_resource.cpp" ]
+    deps = [ "//libcxx/include" ]
     configs += [ ":cxx_config" ]
     configs -= [
       "//llvm/utils/gn/build:no_exceptions",

llvm/utils/gn/secondary/libcxxabi/src/BUILD.gn

@@ -61,7 +61,6 @@ if (target_os == "linux" || target_os == "fuchsia") {
 config("cxxabi_config") {
   include_dirs = [
     "//libcxxabi/include",
-    "//libcxx/include",
 
     # stdlib_stdexcept.cpp depends on libc++ internals.
     "//libcxx",
@@ -89,6 +88,7 @@ if (libcxxabi_enable_shared) {
     public = cxxabi_headers
     deps = [
       "//compiler-rt/lib/builtins",
+      "//libcxx/include",
       "//libunwind/src:unwind_shared",
     ]
     configs += [ ":cxxabi_config" ]
@@ -119,6 +119,7 @@ if (libcxxabi_enable_static) {
     }
     deps = [
       "//compiler-rt/lib/builtins",
+      "//libcxx/include",
       "//libunwind/src:unwind_static",
     ]
     configs += [ ":cxxabi_config" ]