Use scoped_switch_to_sym_language_if_auto in symtab.c to switch language.

Use scoped_switch_to_sym_language_if_auto in treg_matches_sym_type_name to
replace the local logic that was doing the same as the new class
scoped_switch_to_sym_language_if_auto.

Use scoped_switch_to_sym_language_if_auto inside print_symbol_info, so
that symbol information is printed in the symbol language when
language mode is auto.
This modifies the behaviour of the test dw2-case-insensitive.exp,
as the function FUNC_lang is now printed with the Fortran syntax
(as declared in the .S file).

gdb/ChangeLog
2018-11-20  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* symtab.c (treg_matches_sym_type_name): Use
	scoped_switch_to_sym_language_if_auto instead of local logic.
	(print_symbol_info): Use scoped_switch_to_sym_language_if_auto
	to switch to SYM language when language mode is auto.

gdb/testsuite/ChangeLog
2018-11-20  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* gdb.dwarf2/dw2-case-insensitive.exp: Update due to auto switch to
	FUNC_lang language syntax.