[lldb] [windows] Add a [[fallthrough]] to silence warnings. NFC. (#191400)

lldb/source/Host/windows/ProcessLauncherWindows.cpp

diff --git a/lldb/source/Host/windows/ProcessLauncherWindows.cpp b/lldb/source/Host/windows/ProcessLauncherWindows.cpp
index b9cc49d..79e0fac 100644
--- a/lldb/source/Host/windows/ProcessLauncherWindows.cpp
+++ b/lldb/source/Host/windows/ProcessLauncherWindows.cpp
@@ -340,6 +340,7 @@
     break;
   case STDERR_FILENO:
     flags = FILE_FLAG_WRITE_THROUGH;
+    [[fallthrough]];
   case STDOUT_FILENO:
     access = GENERIC_WRITE;
     create = CREATE_ALWAYS;