[gdb] Don't set initial language if set manually

[gdb] Don't set initial language if set manually

Initially, gdb sets the language to auto/c:
...
$ gdb -q
(gdb) show language
The current source language is "auto; currently c".
...

And after loading a c++ executable, that changes to auto/c++:
...
(gdb) file a.out
Reading symbols from a.out...
(gdb) show language
The current source language is "auto; currently c++".
...

Now consider setting the language manually to c:
...
$ gdb -q
(gdb) show language
The current source language is "auto; currently c".
(gdb) set language c
(gdb) show language
The current source language is "c".
...
The resulting language is manual/c.

Surprisingly, a subsequent load of the c++ executable:
...
(gdb) file a.out
Reading symbols from a.out...
(gdb) show language
The current source language is "c++".
...
gets us language manual/c++.

Loading the file should get us either:
- auto/c++, or
- manual/c.
That is, either the manual setting should be reset by loading, or the manual
setting should persist.

Fix this in the manual/c fashion. [ Though we could make some gdb setting to
choose one or the other. ]

Build and reg-tested on x86_64-linux.

[ Note: In PR23710 comment 1 a cc1 binary is attached for which gdb is slow when
loading and settting a breakpoint on do_rpo_vn:
...
$ time.sh gdb cc1 -batch -ex "b do_rpo_vn"
Breakpoint 1 at 0xd40e30: do_rpo_vn. (2 locations)
maxmem: 1463496
real: 8.88
user: 8.59
system: 0.35
...

This fix enables a speedup by manually setting the language before
loading, reducing executing time with ~17%, due to not having to load the full
symtab containing main:
...
$ time.sh gdb -iex "set language c++" cc1 -batch -ex "b do_rpo_vn"
Breakpoint 1 at 0xd40e30: do_rpo_vn. (2 locations)
maxmem: 1067308
real: 7.36
user: 7.14
system: 0.28
... ]

gdb/ChangeLog:

2020-02-26  Tom de Vries  <tdevries@suse.de>

	PR gdb/25603
	* symfile.c (set_initial_language): Exit-early if
	language_mode == language_mode_manual.

gdb/testsuite/ChangeLog:

2020-02-26  Tom de Vries  <tdevries@suse.de>

	PR gdb/25603
	* gdb.base/persistent-lang.cc: New test.
	* gdb.base/persistent-lang.exp: New file.