link-tests: test unwind info emitter via c++ exceptions

test/link/macho/unwind_info/simple_string.cpp

@@ -0,0 +1,30 @@
+#include "all.h"
+#include <cstdio>
+#include <cstring>
+
+SimpleString::SimpleString(size_t max_size)
+: max_size{ max_size }, length{} {
+  if (max_size == 0) {
+    throw Error{ "Max size must be at least 1." };
+  }
+  buffer = new char[max_size];
+  buffer[0] = 0;
+}
+
+SimpleString::~SimpleString() {
+  delete[] buffer;
+}
+
+void SimpleString::print(const char* tag) const {
+  printf("%s: %s", tag, buffer);
+}
+
+bool SimpleString::append_line(const char* x) {
+  const auto x_len = strlen(x);
+  if (x_len + length + 2 > max_size) return false;
+  std::strncpy(buffer + length, x, max_size - length);
+  length += x_len;
+  buffer[length++] = '\n';
+  buffer[length] = 0;
+  return true;
+}