Consolidate CU language setting

Consolidate CU language setting

The DWARF reader currently sets the CU's language in two different
spots.  It is primarily done in prepare_one_comp_unit, but
read_file_scope also checks the producer and may change the language
based on the result.

This patch consolidates all language-setting into
prepare_one_comp_unit.  set_cu_language is renamed and changed not to
set language_defn; instead that is done in prepare_one_comp_unit after
the correct language enum value is chosen.

This fixes a minor latent bug, which is that read_file_scope could set
the language enum value to language_opencl, but then neglected to
reset language_defn in this case.

gdb/ChangeLog
2021-06-25  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (read_file_scope): Don't call set_cu_language.
	(dwarf_lang_to_enum_language): Rename from set_cu_language.  Don't
	set language_defn.  Handle DW_LANG_OpenCL.
	(prepare_one_comp_unit): Check producer and set language_defn.