fix: LLVM frontend candidates need .exe suffix on Windows

toolchain_frontend() checks for "clang++" in the bin dir, but on
Windows the binary is "clang++.exe". Add it to the candidates list.

Author: Sunrisepeak

src/toolchain/llvm.cppm

@@ -24,7 +24,11 @@ std::string package_name() {
 }
 
 std::vector<std::string> frontend_candidates() {
+#if defined(_WIN32)
+    return {"clang++.exe", "clang++"};
+#else
     return {"clang++"};
+#endif
 }
 
 std::vector<std::string> list_aliases() {