c++: Unshare expressions from constexpr cache.

Another place we need to unshare cached expressions.

	PR c++/92852 - ICE with generic lambda and reference var.
	* constexpr.c (maybe_constant_value): Likewise.

gcc/cp/ChangeLog

+2020-01-24  Jason Merrill  <jason@redhat.com>
+
+	PR c++/92852 - ICE with generic lambda and reference var.
+	* constexpr.c (maybe_constant_value): Likewise.
+
 2020-01-23  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	PR c++/92804

gcc/cp/constexpr.c

@@ -6598,7 +6598,7 @@ maybe_constant_value (tree t, tree decl, bool manifestly_const_eval)
   if (cv_cache == NULL)
     cv_cache = hash_map<tree, tree>::create_ggc (101);
   if (tree *cached = cv_cache->get (t))
-    return *cached;
+    return unshare_expr_without_location (*cached);
 
   r = cxx_eval_outermost_constant_expr (t, true, true, false, false, decl);
   gcc_checking_assert (r == t

gcc/testsuite/g++.dg/cpp1y/lambda-generic-ref1.C

+// PR c++/92852
+// { dg-do compile { target c++14 } }
+
+struct S { int operator<<(const int &); } glob;
+void foo()
+{
+  S& message_stream = glob;
+  auto format = [&message_stream](auto && x)
+		{ message_stream << x ; };
+  format(3);
+  format(4u);
+}

gcc/testsuite/g++.dg/cpp1z/decomp48.C

@@ -18,7 +18,7 @@ f2 ()
 {
   S v {1, 2};
   auto& [s, t] = v;	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
-  return s;		// { dg-warning "reference to local variable 'v' returned" "" { target *-*-* } .-1 }
+  return s;		// { dg-warning "reference to local variable 'v' returned" }
 }
 
 int &
@@ -33,7 +33,7 @@ f4 ()
 {
   int a[3] = {1, 2, 3};
   auto& [s, t, u] = a;	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
-  return s;		// { dg-warning "reference to local variable 'a' returned" "" { target *-*-* } .-1 }
+  return s;		// { dg-warning "reference to local variable 'a' returned" }
 }
 
 int &
@@ -78,7 +78,7 @@ f10 ()
 {
   S v {1, 2};
   auto& [s, t] = v;	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
-  return &s;		// { dg-warning "address of local variable 'v' returned" "" { target *-*-* } .-1 }
+  return &s;		// { dg-warning "address of local variable 'v' returned" }
 }
 
 int *
@@ -93,7 +93,7 @@ f12 ()
 {
   int a[3] = {1, 2, 3};
   auto& [s, t, u] = a;	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
-  return &s;		// { dg-warning "address of local variable 'a' returned" "" { target *-*-* } .-1 }
+  return &s;		// { dg-warning "address of local variable 'a' returned" }
 }
 
 int *