[gn] port 0580563

Author: nico

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

@@ -298,6 +298,7 @@ write_cmake_config("llvm-config") {
   values = [
     "LLVM_BUILD_LLVM_DYLIB=",
     "LLVM_BUILD_SHARED_LIBS=",
+    "LLVM_ENABLE_LLVM_C_EXPORT_ANNOTATIONS=",
     "LLVM_ENABLE_TELEMETRY=",
     "LLVM_DEFAULT_TARGET_TRIPLE=$llvm_target_triple",
     "LLVM_ENABLE_DEBUGLOC_TRACKING_COVERAGE=",