gdb/dwarf: add dwarf_source_language_name

Add dwarf_source_language_name, to convert a DW_LANG_* constant to
string.  This will be used in a following patch.

Change-Id: I552ebd318e2e770d590de5920edbd0b75075c1b7
Approved-By: Tom Tromey <tom@tromey.com>
2 files changed