[Frontend] Fix a small unique_ptr array access.

When the size of the array accessed here is zero, retrieving the
address of the zero-th element here is undefined. When the frontend
is linked against a libc++ that has the `unique_ptr` hardening in
[this commit](llvm/llvm-project@18df9d2)
enabled, it traps here.

Instead, simply call `.get()` to retrieve the address of the
array, which works even when it is a zero-byte allocation.

Author: allevato

lib/FrontendTool/FrontendTool.cpp

@@ -1988,7 +1988,7 @@ int swift::performFrontend(ArrayRef<const char *> Args,
   // hundreds or thousands of lines. Skip dumping this output in that case.
   if (!Invocation.getFrontendOptions().InputsAndOutputs.isWholeModule()) {
     for_each(configurationFileBuffers.begin(), configurationFileBuffers.end(),
-             &configurationFileStackTraces[0],
+             configurationFileStackTraces.get(),
              [](const std::unique_ptr<llvm::MemoryBuffer> &buffer,
                 std::optional<PrettyStackTraceFileContents> &trace) {
                trace.emplace(*buffer);