[swift-stdlib-tool] Add error for missing directories

In the case the user passes a directory that doesn't exist, this now
errors immediately instead of silently ignoring it.

Author: keith

tools/swift-stdlib-tool/swift-stdlib-tool.cpp

@@ -728,6 +728,11 @@ std::string filename(std::string path) {
   return basename_r(pathCstr, filename) ? filename : pathCstr;
 }
 
+bool directory_exists(const std::string &path) {
+  struct stat st;
+  return stat(path.c_str(), &st) == 0 && S_ISDIR(st.st_mode);
+}
+
 // This executable's own path.
 std::string self_executable = []() -> std::string {
   char path[MAXPATHLEN] = {0};
@@ -1115,6 +1120,11 @@ int main(int argc, const char *argv[]) {
     platform = filename(src_dirs.front());
   }
 
+  for (const auto &src_dir : src_dirs) {
+    if (!directory_exists(src_dir))
+      fail("Source directory does not exist: %s", src_dir.c_str());
+  }
+
   // Add the platform to unsigned_dst_dir if it is not already present.
   if (!unsigned_dst_dir.empty()) {
     const auto unsigned_platform = unsigned_dst_dir;