feat: Clang on Windows — find MSVC STL std.ixx for import std support

When Clang targets x86_64-pc-windows-msvc, it uses MSVC STL (not
libc++). Add fallback search for Visual Studio's std.ixx in the
modules/ directory. Also add .exe suffix for clang-scan-deps and
llvm-ar on Windows.

Author: Sunrisepeak

src/toolchain/clang.cppm

@@ -136,14 +136,44 @@ std::optional<std::filesystem::path> find_libcxx_std_module_source(
 }
 
 void enrich_toolchain(Toolchain& tc, const std::string& envPrefix) {
-    tc.stdlibId      = "libc++";
+    // Clang targeting MSVC uses MSVC STL, not libc++.
+    bool msvTarget = tc.targetTriple.find("msvc") != std::string::npos;
+    tc.stdlibId      = msvTarget ? "msvc-stl" : "libc++";
     tc.stdlibVersion = tc.version.empty() ? "unknown" : tc.version;
     tc.linkRuntimeDirs = mcpp::toolchain::discover_link_runtime_dirs(
         tc.binaryPath, tc.targetTriple);
+
     if (auto p = find_libcxx_std_module_source(tc.binaryPath, envPrefix)) {
         tc.stdModuleSource = *p;
         tc.hasImportStd    = true;
     }
+
+#if defined(_WIN32)
+    // Fallback: if libc++ std.cppm not found, look for MSVC STL's std.ixx.
+    // This happens when Clang targets x86_64-pc-windows-msvc.
+    if (!tc.hasImportStd && msvTarget) {
+        // Search Visual Studio installations for std.ixx
+        // Typical path: C:\Program Files\Microsoft Visual Studio\2022\*\VC\Tools\MSVC\*\modules\std.ixx
+        std::error_code ec;
+        std::filesystem::path vsBase = "C:\\Program Files\\Microsoft Visual Studio\\2022";
+        if (std::filesystem::exists(vsBase, ec)) {
+            for (auto& edition : std::filesystem::directory_iterator(vsBase, ec)) {
+                auto vcTools = edition.path() / "VC" / "Tools" / "MSVC";
+                if (!std::filesystem::exists(vcTools, ec)) continue;
+                for (auto& ver : std::filesystem::directory_iterator(vcTools, ec)) {
+                    auto stdIxx = ver.path() / "modules" / "std.ixx";
+                    if (std::filesystem::exists(stdIxx, ec)) {
+                        tc.stdModuleSource = stdIxx;
+                        tc.hasImportStd    = true;
+                        break;
+                    }
+                }
+                if (tc.hasImportStd) break;
+            }
+        }
+    }
+#endif
+
     if (tc.hasImportStd) {
         if (auto p = find_libcxx_std_compat_source(tc.binaryPath, envPrefix)) {
             tc.stdCompatSource = *p;
@@ -186,13 +216,21 @@ std::vector<std::string> std_module_build_commands(const Toolchain& tc,
 }
 
 std::filesystem::path archive_tool(const Toolchain& tc) {
+#if defined(_WIN32)
+    auto llvmAr = tc.binaryPath.parent_path() / "llvm-ar.exe";
+#else
     auto llvmAr = tc.binaryPath.parent_path() / "llvm-ar";
+#endif
     if (std::filesystem::exists(llvmAr)) return llvmAr;
     return {};
 }
 
 std::optional<std::filesystem::path> find_scan_deps(const Toolchain& tc) {
+#if defined(_WIN32)
+    auto p = tc.binaryPath.parent_path() / "clang-scan-deps.exe";
+#else
     auto p = tc.binaryPath.parent_path() / "clang-scan-deps";
+#endif
     if (std::filesystem::exists(p)) return p;
     return std::nullopt;
 }