[GWP-ASan] Fix sanitizer backtrace util using wrong print.

[hctim]

Summary

The sanitizer backtrace is used in tests, and calling StackTrace.Print()
means that it uses the sanitizer's Printf(), rather than GWP-ASan's
Printf(). In the current code, GWP-ASan's Printf() *is* the sanitizer
print, but this isn't guaranteed to be the case, and will change in an
upcoming patch.

[eugenis]

LGTM

compiler-rt/lib/gwp_asan/optional/backtrace_sanitizer_common.cpp

static void PrintBacktrace(uintptr_t *Trace, size_t TraceLength,
   StackTrace.trace = reinterpret_cast<__sanitizer::uptr *>(Trace);
   StackTrace.size = TraceLength;
 
   if (StackTrace.size == 0) {
     Printf("  <unknown (does your allocator support backtracing?)>\n\n");
     return;
   }
 
-  StackTrace.Print();
+  __sanitizer::InternalScopedString buffer;
+  StackTrace.PrintTo(&buffer);
+  Printf("%s\n", buffer.data());
 }
 } // anonymous namespace
 
 namespace gwp_asan {
 namespace backtrace {
 
 // This function is thread-compatible. It must be synchronised in respect to any
 // other calls to getBacktraceFunction(), calls to getPrintBacktraceFunction(),