libiberty: move DW_LANG_* definitions to dwarf2.def

In order to get a "DW_LANG_* to string" function:

  - move the "DW_LANG_*" definitions from dwarf2.h to dwarf2.def
  - add the necessary macros in dwarf2.h to generate the enumeration
  - add the necessary macros in dwarfnames.c to generate the "to string"
    function

include/ChangeLog:

	* dwarf2.h (DW_LANG, DW_FIRST_LANG, DW_END_LANG): Define then
	undefine.
	(enum dwarf_source_language): Remove.
	(get_DW_LANG_name): Declare.
	* dwarf2.def: Define DW_LANG_* constants.

libiberty/ChangeLog:

	* dwarfnames.c (DW_FIRST_LANG, DW_END_LANG, DW_LANG): Define
	then undefine.

Change-Id: I440aa2b1f55c7585d7e44c8fa7c41310b0ef2b3a
Cc: binutils@sourceware.org