[gdb/symtab] Make per_cu->set_lang more strict

[gdb/symtab] Make per_cu->set_lang more strict

We have in per_cu->set_lang this comment:
...
  void set_lang (enum language lang)
  {
    /* We'd like to be more strict here, similar to what is done in
       set_unit_type,  but currently a partial unit can go from unknown to
       minimal to ada to c.  */
...

Fix this by not setting m_lang for partial units.

This requires us to move the m_unit_type initialization to ensure that
m_unit_type is initialized before per_cu->m_lang.

Tested on x86_64-linux, with native and target board cc-with-dwz-m.