gdb: silence unused-but-set-variable warning about yynerrs in cp-name-parser.y

gdb: silence unused-but-set-variable warning about yynerrs in cp-name-parser.y

When building with clang 15 on Ubuntu 20.04, I get:

      CXX    cp-name-parser.o
    cp-name-parser.c.tmp:1777:9: error: variable 'cpnameyynerrs' set but not used [-Werror,-Wunused-but-set-variable]
        int yynerrs;
            ^
    /home/smarchi/src/binutils-gdb/gdb/yy-remap.h:58:18: note: expanded from macro 'yynerrs'
    #define yynerrs         GDB_YY_REMAP (yynerrs)
                            ^
    /home/smarchi/src/binutils-gdb/gdb/yy-remap.h:40:29: note: expanded from macro 'GDB_YY_REMAP'
    #define GDB_YY_REMAP(YYSYM) GDB_YY_REMAP_1 (GDB_YY_REMAP_PREFIX, YYSYM)
                                ^
    /home/smarchi/src/binutils-gdb/gdb/yy-remap.h:39:39: note: expanded from macro 'GDB_YY_REMAP_1'
    #define GDB_YY_REMAP_1(PREFIX, YYSYM) GDB_YY_REMAP_2 (PREFIX, YYSYM)
                                          ^
    /home/smarchi/src/binutils-gdb/gdb/yy-remap.h:38:39: note: expanded from macro 'GDB_YY_REMAP_2'
    #define GDB_YY_REMAP_2(PREFIX, YYSYM) PREFIX ## YYSYM
                                          ^
    <scratch space>:45:1: note: expanded from here
    cpnameyynerrs
    ^

This is because clang 15 warns for something like this:

    int n;
    n = 0;
    ++n;

whereas previous versions do not.

yynerrs is defined in yyparse and is there for actions to use.  Since
the actions in cp-name-parser.y don't use it, we get a warning.  We see
this problem on this particular .y file because it uses `%pure-parser`
[1], which makes yynerrs a local rather than a global.

I initially fixed this by using
DIAGNOSTIC_IGNORE_UNUSED_BUT_SET_VARIABLE (like in commit f7aa1a5acc5
("gold: Suppress "unused" variable warning on Clang")), but then I
realized we could suppress the warning in a more fine-grained way using
this in a rule:

    (void) yynerrs;

[1] https://www.gnu.org/software/bison/manual/html_node/Error-Reporting-Function.html

Change-Id: I6cae7a4207c19fe1b719e2ac19be69122ebe3af1