Currently, drgn_language_from_die() returns the default language when it
encounters an unknown DW_LANG because the dwarf_info_cache always wants
a language. The next change will want to detect the unknown language
case, so make drgn_language_from_die() return NULL if the language is
unknown, move it to language.c, and fold drgn_language_from_dw_lang()
into it.
For types obtained from DWARF, we determine it from the language of the
CU. For other types, it can be specified manually or fall back to the
default (C). Then, we can use the language for operations where the type
is available.
This way, languages can be identified by an index, which will be useful
for adding Python bindings for drgn_language and for adding a language
field to drgn_type.
