Remove la_error

While working on the parser code, I noticed that yyerror is exported
from each parser.  It is used by this code in parse.c:

   TRY
     {
       if (lang->la_parser (&ps))
         lang->la_error (NULL);
     }

However, it seems to me that la_error will never be called here,
because in every case, la_parser throws an exception on error -- each
implementation of yyerror just calls error.

So, this patch removes la_error and makes all the yyerror functions
static.  This is handy primarily because it makes it simpler to make
the expression parsers pure.

Tested by the buildbot.

gdb/ChangeLog
2018-06-18  Tom Tromey  <tom@tromey.com>

	* rust-lang.h (rust_yyerror): Don't declare.
	* rust-lang.c (rust_language_defn): Update.
	* rust-exp.y (yyerror): Now static.
	* parse.c (parse_exp_in_context_1): Update.
	* p-lang.h (p_yyerror): Don't declare.
	* p-lang.c (p_language_defn): Update.
	* p-exp.y (yyerror): Now static.
	* opencl-lang.c (opencl_language_defn): Update.
	* objc-lang.c (objc_language_defn): Update.
	* m2-lang.h (m2_yyerror): Don't declare.
	* m2-lang.c (m2_language_defn): Update.
	* m2-exp.y (yyerror): Now static.
	* language.h (struct language_defn) <la_error>: Remove.
	* language.c (unk_lang_error): Remove.
	(unknown_language_defn, auto_language_defn): Remove.
	* go-lang.h (go_yyerror): Don't declare.
	* go-lang.c (go_language_defn): Update.
	* go-exp.y (yyerror): Now static.
	* f-lang.h (f_yyerror): Don't declare.
	* f-lang.c (f_language_defn): Update.
	* f-exp.y (yyerror): Now static.
	* d-lang.h (d_yyerror): Don't declare.
	* d-lang.c (d_language_defn): Update.
	* d-exp.y (yyerror): Now static.
	* c-lang.h (c_yyerror): Don't declare.
	* c-lang.c (c_language_defn, cplus_language_defn)
	(asm_language_defn, minimal_language_defn): Update.
	* c-exp.y (yyerror): Now static.
	* ada-lang.h (ada_yyerror): Don't declare.
	* ada-lang.c (ada_language_defn): Update.
	* ada-exp.y (yyerror): Now static.