gn build: libcxx: Add libc include path and re-enable.

Reviewers: aeubanks, nico

Reviewed By: aeubanks

Pull Request: #124679

Author: pcc

llvm/utils/gn/secondary/BUILD.gn

@@ -14,8 +14,7 @@ group("default") {
     "//lldb/test",
     "//llvm/test",
   ]
-  # FIXME: Reenable once libc hand-in-hand is figured out.
-  if (false && current_os == "linux") {
+  if (current_os == "linux") {
     deps += [
       "//libcxx",
       "//libcxxabi",

llvm/utils/gn/secondary/libc/BUILD.gn

@@ -0,0 +1,8 @@
+config("common_utils_config") {
+  include_dirs = [ "." ]
+  defines = [ "LIBC_NAMESPACE=__llvm_libc_common_utils" ]
+}
+
+group("common_utils") {
+  public_configs = [ ":common_utils_config" ]
+}

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

@@ -247,6 +247,7 @@ if (libcxx_enable_shared) {
     include_dirs = [ "." ]
     deps = [
       "//compiler-rt/lib/builtins",
+      "//libc:common_utils",
       "//libcxx/include",
       "//libcxxabi/src:cxxabi_shared",
       "//libunwind/src:unwind_shared",
@@ -298,6 +299,7 @@ if (libcxx_enable_static) {
     }
     deps = [
       "//compiler-rt/lib/builtins",
+      "//libc:common_utils",
       "//libcxx/include",
       "//libcxxabi/src:cxxabi_static",
       "//libunwind/src:unwind_static",