Fix language of compilation unit with unknown file extension

Fix language of compilation unit with unknown file extension

Here, in dwarfread.c:process_full_comp_unit:

      /* Set symtab language to language from DW_AT_language.  If the
	 compilation is from a C file generated by language preprocessors, do
	 not set the language if it was already deduced by start_subfile.  */
      if (!(cu->language == language_c
	    && COMPUNIT_FILETABS (cust)->language != language_c))
	COMPUNIT_FILETABS (cust)->language = cu->language;

in case start_subfile doesn't manage to deduce a language
COMPUNIT_FILETABS(cust)->language ends up as language_unknown, not
language_c.  So the condition above evals false and we never set the
language from the cu's language.

gdb/ChangeLog:
2015-08-20  Pedro Alves  <palves@redhat.com>

	* dwarf2read.c (process_full_comp_unit): To tell whether
	start_subfile managed to deduce a language, test for
	language_unknown instead of language_c.

gdb/testsuite/ChangeLog:
2015-08-20  Pedro Alves  <palves@redhat.com>

	* gdb.dwarf2/comp-unit-lang.exp: New file.
	* gdb.dwarf2/comp-unit-lang.c: New file.