compiler: make comparison operator() methods const

    
    This is required for new versions of libstdc++ in C++17 mode.
    
    Fixes GCC PR 83102.
    
    Reviewed-on: https://go-review.googlesource.com/79396

From-SVN: r255062

gcc/go/gofrontend/MERGE

-178c00f929e6268f24f0c9dd9eb69d3d14d8096e
+1db7dc97d01ee230ff4874ce1c9775a24ffd16ac
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/gogo.h

@@ -117,7 +117,7 @@ class Import_init
 // For sorting purposes.
 
 struct Import_init_lt {
-  bool operator()(const Import_init* i1, const Import_init* i2)
+  bool operator()(const Import_init* i1, const Import_init* i2) const
   {
     return i1->init_name() < i2->init_name();
   }

gcc/go/gofrontend/parse.cc

@@ -20,7 +20,7 @@
 
 bool
 Parse::Enclosing_var_comparison::operator()(const Enclosing_var& v1,
-					    const Enclosing_var& v2)
+					    const Enclosing_var& v2) const
 {
   if (v1.var() == v2.var())
     return false;

gcc/go/gofrontend/parse.h

@@ -127,7 +127,7 @@ class Parse
   struct Enclosing_var_comparison
   {
     bool
-    operator()(const Enclosing_var&, const Enclosing_var&);
+    operator()(const Enclosing_var&, const Enclosing_var&) const;
   };
 
   // A set of Enclosing_var entries.