[gdb/symtab] Add dwarf2_cu::lang ()

[gdb/symtab] Add dwarf2_cu::lang ()

The cu->per_cu->lang field was added to carry information from the initial
partial symtabs phase to the symtab expansion phase, for the benefit of a
particular optimization in process_imported_unit_die.

Other uses have been added, but since the first phase now has been
parallelized, those have become problematic and sources of race conditions.

Fix this by adding dwarf2_cu::lang () and using it where we can to replace
cu->per_cu->lang () with cu->lang ().

Also assert in dwarf2_cu::lang () that we're not returning language_unknown.

Tested on x86_64-linux.