Avoid expression parsing crash with unknown language

Avoid expression parsing crash with unknown language

PR gdb/28093 points out that gdb crashes when language is set to
"unknown" and expression parsing is attempted.  At first I thought
this was a regression due to the expression rewrite, but it turns out
that older versions crash as well.

This patch avoids the crash by changing the default expression parser
to throw an exception.  I think this is preferable -- the current
behavior of silently doing nothing does not really make sense.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28093
(cherry picked from commit dcd482c1b756d9da2130552a6eb58b852d6efb97)

gdb/ChangeLog
2021-07-19  Tom Tromey  <tromey@adacore.com>

	PR gdb/28093
	* language.c (auto_or_unknown_language::parser): Call error.

gdb/testsuite/ChangeLog
2021-07-19  Tom Tromey  <tromey@adacore.com>

	PR gdb/28093
	* gdb.base/langs.exp: Add tests.