fix: bypass stale clang++.cfg for macOS std module precompile

When xlings copies LLVM to mcpp's sandbox, clang++.cfg retains
hardcoded absolute paths from the original install (--sysroot and
-isystem pointing to ~/.xlings/... instead of ~/.mcpp/...). These
stale paths cause _CTYPE_A undeclared errors during std module
precompilation.

Fix: on macOS Clang, pass --no-default-config to ignore the stale
cfg, then explicitly provide the correct -isystem (libc++ headers
from the sandbox LLVM root) and --sysroot (active SDK from xcrun).
This produces the same header search behavior as a fresh clang++
invocation with the correct paths.

Author: Sunrisepeak

src/toolchain/stdmod.cppm

@@ -92,21 +92,27 @@ std::expected<StdModule, StdModError> ensure_built(
                   : mcpp::toolchain::gcc::std_bmi_path(sm.cacheDir);
     sm.objectPath = sm.cacheDir / "std.o";
 
+    // Build sysroot + include flags for std module precompilation.
+    // On macOS, xlings LLVM's clang++.cfg contains hardcoded --sysroot and
+    // -isystem paths from the original install location. When the LLVM package
+    // is copied to mcpp's sandbox, these cfg paths become stale (still point
+    // to the original xlings directory). We override both:
+    //   --sysroot   → current active SDK (from xcrun)
+    //   --no-default-config  → ignore stale cfg entirely
+    //   -isystem    → correct libc++ headers in the sandbox copy
     std::string sysroot_flag;
-    if (!tc.sysroot.empty()) {
-        // macOS (apple/darwin target): do NOT pass --sysroot for std module
-        // precompilation. xlings LLVM's clang++.cfg already contains a
-        // --sysroot for the macOS SDK plus -isystem for libc++ headers.
-        // Passing an explicit --sysroot on the command line (even the same
-        // value) changes Clang's internal header search order, causing the
-        // macOS SDK's ___wctype.h to not find _CTYPE_A (defined in a
-        // sibling C runtime header that's only included transitively via
-        // the cfg's default search path). The CI "import std" test proves
-        // that running clang++ WITHOUT an explicit --sysroot works correctly.
-        bool is_macos = tc.targetTriple.find("apple") != std::string::npos
-                     || tc.targetTriple.find("darwin") != std::string::npos;
-        if (!is_macos)
-            sysroot_flag = std::format(" --sysroot='{}'", tc.sysroot.string());
+    bool is_macos = tc.targetTriple.find("apple") != std::string::npos
+                 || tc.targetTriple.find("darwin") != std::string::npos;
+    if (is_macos && is_clang(tc)) {
+        // Ignore the stale clang++.cfg and provide correct flags directly.
+        auto llvmRoot = tc.binaryPath.parent_path().parent_path();
+        auto libcxxInclude = llvmRoot / "include" / "c++" / "v1";
+        sysroot_flag = " --no-default-config";
+        sysroot_flag += std::format(" -isystem'{}'", libcxxInclude.string());
+        if (auto sdk = mcpp::platform::macos::sdk_path())
+            sysroot_flag += std::format(" --sysroot='{}'", sdk->string());
+    } else if (!tc.sysroot.empty()) {
+        sysroot_flag = std::format(" --sysroot='{}'", tc.sysroot.string());
     }
 
     bool std_cached = std::filesystem::exists(sm.bmiPath) && std::filesystem::exists(sm.objectPath);