[lldb] Log when we use fallback register information

These fallback layouts are essentially guesses. Used when there is
no other way to query register information from the debug server.

Therefore there is a risk that LLDB and the debug server disagree,
which can produce strange effects.

I have added a log message here so we have a clue when triaging
these problems.

Note that it's not wrong to assume a layout in some situations.
It's how some debug servers were built. However if you end up
using the fallback when the server expected you to use XML,
you're likely going to have a bad time.

Author: DavidSpickett

lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp

@@ -570,8 +570,13 @@ void ProcessGDBRemote::BuildDynamicRegisterInfo(bool force) {
     }
   }
 
-  if (registers.empty())
+  if (registers.empty()) {
     registers = GetFallbackRegisters(arch_to_use);
+    if (!registers.empty())
+      LLDB_LOG(
+          log,
+          "All other methods failed, using fallback register information.");
+  }
 
   AddRemoteRegisters(registers, arch_to_use);
 }