c++: use noexcept where appropriate

Hi Don,

Thanks for the reports.  I will install this.  Cheers!

commit 9840e5b621be661c2f7cd593a8bcb910447531da
Author: Akim Demaille <[email protected]>
Date:   Wed Dec 2 21:38:43 2020 +0100

    c++: use noexcept where appropriate
    
    Reported by Don Macpherson.
    https://github.com/akimd/bison/issues/63
    https://github.com/akimd/bison/issues/64
    
    * data/skeletons/c++.m4, data/skeletons/lalr1.cc: here.

diff --git a/data/skeletons/c++.m4 b/data/skeletons/c++.m4
index f4834c64..b458af3f 100644
--- a/data/skeletons/c++.m4
+++ b/data/skeletons/c++.m4
@@ -339,7 +339,7 @@ m4_define([b4_symbol_type_define],
       }
 
       /// Destroy contents, and record that is empty.
-      void clear ()
+      void clear () YY_NOEXCEPT
       {]b4_variant_if([[
         // User destructor.
         symbol_kind_type yykind = this->kind ();
@@ -423,7 +423,7 @@ m4_define([b4_symbol_type_define],
       by_kind (kind_type t);
 
       /// Record that this symbol is empty.
-      void clear ();
+      void clear () YY_NOEXCEPT;
 
       /// Steal the symbol kind from \a that.
       void move (by_kind& that);
@@ -543,7 +543,7 @@ m4_define([b4_public_types_define],
   {}
 
   ]b4_inline([$1])[void
-  ]b4_parser_class[::by_kind::clear ()
+  ]b4_parser_class[::by_kind::clear () YY_NOEXCEPT
   {
     kind_ = ]b4_symbol(empty, kind)[;
   }
diff --git a/data/skeletons/lalr1.cc b/data/skeletons/lalr1.cc
index fbabba35..76603492 100644
--- a/data/skeletons/lalr1.cc
+++ b/data/skeletons/lalr1.cc
@@ -270,9 +270,9 @@ m4_define([b4_shared_declarations],
     {
     public:
       context (const ]b4_parser_class[& yyparser, const symbol_type& yyla);
-      const symbol_type& lookahead () const { return yyla_; }
-      symbol_kind_type token () const { return yyla_.kind (); }]b4_locations_if([[
-      const location_type& location () const { return yyla_.location; }
+      const symbol_type& lookahead () const YY_NOEXCEPT { return yyla_; }
+      symbol_kind_type token () const YY_NOEXCEPT { return yyla_.kind (); }]b4_locations_if([[
+      const location_type& location () const YY_NOEXCEPT { return yyla_.location; }
 ]])[
       /// Put in YYARG at most YYARGN of the expected tokens, and return the
       /// number of tokens stored in YYARG.  If YYARG is null, return the