llvm-native: use provided tblgen, don't force C++11 native sub-build

Two fixes for the native tblgen sub-build failing on <shared_mutex>:
- LLVM_OPTIMIZED_TABLEGEN=Off: this recipe already provides a host-runnable,
  version-matched llvm-/clang-tblgen (redoxer toolchain, LLVM 21.1.2) via
  *_TABLEGEN_EXE. OPTIMIZED_TABLEGEN=On ignored those and forced a redundant
  native sub-build (CROSS_TOOLCHAIN_FLAGS_NATIVE) that failed. Off => LLVM uses
  the provided tblgen and skips the native build. (llvm21 provides no tblgen,
  so it keeps On and builds its own — different case, left as-is.)
- CMAKE_CXX_FLAGS --std=gnu++11 -> gnu++17: LLVM 21 requires C++17; the stale
  gnu++11 could win the flag-order race in any sub-build and break the headers.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

local/recipes/dev/llvm-native/recipe.toml

@@ -93,7 +93,14 @@ COOKBOOK_CMAKE_FLAGS+=(
     -DLLVM_ENABLE_THREADS=On
     -DLLVM_INCLUDE_EXAMPLES=Off
     -DLLVM_INCLUDE_TESTS=Off
-    -DLLVM_OPTIMIZED_TABLEGEN=On
+    # OFF on purpose: this recipe PROVIDES a host-runnable, version-matched
+    # tblgen via LLVM_TABLEGEN_EXE/CLANG_TABLEGEN_EXE below (the redoxer
+    # toolchain's LLVM 21.1.2). LLVM_OPTIMIZED_TABLEGEN=On would ignore those
+    # and force a redundant native sub-build (CROSS_TOOLCHAIN_FLAGS_NATIVE)
+    # that failed to compile <shared_mutex>. With it off, LLVM uses the
+    # provided tblgen and skips the native build entirely. (llvm21, which does
+    # NOT provide a tblgen, keeps it On and builds its own — different case.)
+    -DLLVM_OPTIMIZED_TABLEGEN=Off
     -DLLVM_TARGET_ARCH=$ARCH
     -DLLVM_TOOLS_INSTALL_DIR=bin
     -DLLVM_UTILS_INSTALL_DIR=bin
@@ -115,10 +122,17 @@ fi
 if [ -n "${_SAVED_CFLAGS}" ]; then
     COOKBOOK_CMAKE_FLAGS+=(-DCMAKE_C_FLAGS="${_SAVED_CFLAGS}")
 fi
+# LLVM 21 REQUIRES C++17 (its headers use std::remove_reference_t, std::size,
+# std::shared_timed_mutex, etc.). The main build gets -std=gnu++17 from LLVM's
+# own CMAKE_CXX_STANDARD=17, appended after CMAKE_CXX_FLAGS so it wins — but the
+# CROSS_TOOLCHAIN_FLAGS_NATIVE sub-build (native llvm-tblgen) inherits
+# CMAKE_CXX_FLAGS with a DIFFERENT flag order where a stale "--std=gnu++11"
+# would win, breaking <shared_mutex> ("'_M_rwlock' was not declared"). Pass
+# gnu++17 here so no target can regress to C++11 regardless of flag order.
 if [ -n "${_SAVED_CXXFLAGS}" ]; then
-    COOKBOOK_CMAKE_FLAGS+=(-DCMAKE_CXX_FLAGS="--std=gnu++11 ${_SAVED_CXXFLAGS}")
+    COOKBOOK_CMAKE_FLAGS+=(-DCMAKE_CXX_FLAGS="--std=gnu++17 ${_SAVED_CXXFLAGS}")
 else
-    COOKBOOK_CMAKE_FLAGS+=(-DCMAKE_CXX_FLAGS="--std=gnu++11")
+    COOKBOOK_CMAKE_FLAGS+=(-DCMAKE_CXX_FLAGS="--std=gnu++17")
 fi
 
 cookbook_cmake