C++ PATCH to allow copying of local_specializations

[Jason Merrill]

When instantiating a lambda-expression in the new model, we want to
retain the local_specializations from the enclosing function but still
be able to remember specializations local to the lambda and discard
them afterwards.  With this patch a local_specialization_stack object
can start with a copy of the enclosing map.

Tested x86_64-pc-linux-gnu, applying to trunk.
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commit aa72ed613b20715d171287c643f611b7c30b3f12
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Jul 11 17:32:21 2017 -0400

            Support copying local_specializations.
    
            * cp-tree.h (enum lss_policy): New.
            (local_specialization_stack): Add policy parameter to default ctor.
            * pt.c (local_specialization_stack): Copy local_specializations if
            lss_copy.

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index f0eafb3..a58e7bd 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -5117,9 +5117,10 @@ enum unification_kind_t {
 // An RAII class used to create a new pointer map for local
 // specializations. When the stack goes out of scope, the
 // previous pointer map is restored.
+enum lss_policy { lss_blank, lss_copy };
 struct local_specialization_stack
 {
-  local_specialization_stack ();
+  local_specialization_stack (lss_policy = lss_blank);
   ~local_specialization_stack ();
 
   hash_map<tree, tree> *saved;
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 847cd68..416d17b 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -77,10 +77,13 @@ static tree cur_stmt_expr;
 //
 // Implementation of the RAII helper for creating new local
 // specializations.
-local_specialization_stack::local_specialization_stack ()
+local_specialization_stack::local_specialization_stack (lss_policy policy)
   : saved (local_specializations)
 {
-  local_specializations = new hash_map<tree, tree>;
+  if (policy == lss_blank || !saved)
+    local_specializations = new hash_map<tree, tree>;
+  else
+    local_specializations = new hash_map<tree, tree>(*saved);
 }
 
 local_specialization_stack::~local_specialization_stack ()