Lazy language setting

Lazy language setting

When gdb starts up with a symbol file, it uses the program's "main" to
decide the "static context" and the initial language.  With background
DWARF reading, this means that gdb has to wait for a significant
amount of DWARF to be read synchronously.

This patch introduces lazy language setting.  The idea here is that in
many cases, the prompt can show up early, making gdb feel more
responsive.