Fixup log enable --stack so it works on Linux.

The underlying raw_string_stream buffer was not being flushed
after asking llvm to collect the backtrace.  This worked fine
on OS X but was failing to print anything on Linux.

llvm-svn: 249930
diff --git a/lldb/source/Core/Log.cpp b/lldb/source/Core/Log.cpp
index 5a3f374..8d415bd 100644
--- a/lldb/source/Core/Log.cpp
+++ b/lldb/source/Core/Log.cpp
@@ -143,6 +143,7 @@
             std::string back_trace;
             llvm::raw_string_ostream stream(back_trace);
             llvm::sys::PrintStackTrace(stream);
+            stream.flush();
             header.PutCString(back_trace.c_str());
         }