[gdb] Don't set initial language using previous language

[gdb] Don't set initial language using previous language

When language is set to auto, part of loading an executable is to update the
language accordingly.  This is implemented by set_initial_language.

In case of a c++ executable without DW_AT_main_subprogram,
set_initial_language finds "main" in the minimal symbols, and does a lookup of
"main" in the symbol tables to determine the language of the symbol, and uses
that as initial language.

The symbol lookup is done using lookup_symbol which is a wrapper around
lookup_symbol_in_language, using the current language.

So, consider two c++ executables a.out and b.out, which we'll load one after
another.  If we track the resulting lookup_symbol_in_language calls:
...
$ gdb -batch \
    -ex "b lookup_symbol_in_language" \
    -ex r -ex c -ex c \
    --args gdb
...
we find that indeed lookup_symbol_in_language is called once using language_c, and
once using language_c_plus:
...
(gdb) file a.out
Reading symbols from a.out...

Breakpoint 1, lookup_symbol_in_language (name=0x5555568c2050 "main", \
  block=0x0, domain=VAR_DOMAIN, lang=language_c, is_a_field_of_this=0x0) \
  at ../../gdb/symtab.c:1905
1905    {
(gdb) file b.out
Load new symbol table from "b.out"? (y or n) y
Reading symbols from b.out...

Breakpoint 1, lookup_symbol_in_language (name=0x5555568c2030 "main", \
  block=0x0, domain=VAR_DOMAIN, lang=language_cplus, is_a_field_of_this=0x0) \
  at ../../gdb/symtab.c:1905
1905    {
(gdb)
...

It seems like a bad idea to have the previous language play a role
in determining the executable language.

Fix this by using lookup_symbol_in_language in set_initial_language with the
default language c as argument.

Tested on x86_64-linux.

gdb/ChangeLog:

2020-02-28  Tom de Vries  <tdevries@suse.de>

	* symfile.c (set_initial_language): Use default language for lookup.