[gdb/symtab] Add get/set functions for per_cu->lang/unit_type

The dwarf2_per_cu_data fields lang and unit_type both have a dont-know
initial value (respectively language_unknown and (dwarf_unit_type)0), which
allows us to add certain checks, f.i. checking that that a field is not read
before written.

Add get/set member functions for the two fields as a convenient location to
add such checks, make the fields private to enforce using the member
functions, and add the m_ prefix.

Tested on x86_64-linux.