Add gdb::hash_enum

The DWARF-5 .debug_names consumer patch will want to use an
std::unordered_map with an enum as key type, like:

	std::unordered_map<sect_offset, dwarf2_per_cu_data*>

That doesn't work in C++11 in non-recent compilers due to a language
defect:

 http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2148

~~~
  In file included from /usr/include/c++/5.3.1/bits/hashtable.h:35:0,
		   from /usr/include/c++/5.3.1/unordered_set:47,
		   from src/gdb/dwarf2read.c:79:
  /usr/include/c++/5.3.1/bits/hashtable_policy.h: In instantiation of ‘struct std::__detail::__is_noexcept_hash<sect_offset, std::hash<sect_offset> >’:
  /usr/include/c++/5.3.1/type_traits:137:12:   required from ‘struct std::__and_<std::__is_fast_hash<std::hash<sect_offset> >, std::__detail::__is_noexcept_hash<sect_offset, std::hash<sect_offset> > >’
  /usr/include/c++/5.3.1/type_traits:148:38:   required from ‘struct std::__not_<std::__and_<std::__is_fast_hash<std::hash<sect_offset> >, std::__detail::__is_noexcept_hash<sect_offset, std::hash<sect_offset> > > >’
  /usr/include/c++/5.3.1/bits/unordered_map.h:100:66:   required from ‘class std::unordered_map<sect_offset, dwarf2_per_cu_data*>’
  src/gdb/dwarf2read.c:3260:30:   required from here
  /usr/include/c++/5.3.1/bits/hashtable_policy.h:85:34: error: no match for call to ‘(const std::hash<sect_offset>) (const sect_offset&)’
    noexcept(declval<const _Hash&>()(declval<const _Key&>()))>
				    ^
  In file included from /usr/include/c++/5.3.1/bits/move.h:57:0,
		   from /usr/include/c++/5.3.1/bits/stl_pair.h:59,
		   from /usr/include/c++/5.3.1/bits/stl_algobase.h:64,
		   from /usr/include/c++/5.3.1/bits/char_traits.h:39,
		   from /usr/include/c++/5.3.1/string:40,
		   from /home/pedro/gdb/mygit/src/gdb/common/common-utils.h:23,
		   from /home/pedro/gdb/mygit/src/gdb/common/common-defs.h:78,
		   from /home/pedro/gdb/mygit/src/gdb/defs.h:28,
		   from /home/pedro/gdb/mygit/src/gdb/dwarf2read.c:31:
~~~

This commits adds a helper replacement.

gdb/ChangeLog:
2017-12-08  Pedro Alves  <palves@redhat.com>

	* common/hash_enum.h: New file.

2 files changed, 49 insertions, 0 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 5d596f0..09cafd0 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2017-12-08  Pedro Alves  <palves@redhat.com>
+
+	* common/hash_enum.h: New file.
+
 2017-12-08  Jan Kratochvil  <jan.kratochvil@redhat.com>
 
 	* dwarf2read.c (create_cu_from_index_list): New from ...
diff --git a/gdb/common/hash_enum.h b/gdb/common/hash_enum.h
new file mode 100644
index 0000000..0ea3221
--- /dev/null
+++ b/gdb/common/hash_enum.h
@@ -0,0 +1,45 @@
+/* A hasher for enums.
+
+   Copyright (C) 2017 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef COMMON_HASH_ENUM_H
+#define COMMON_HASH_ENUM_H
+
+/* A hasher for enums, which was missing in C++11:
+    http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2148
+*/
+
+namespace gdb {
+
+/* Helper struct for hashing enum types.  */
+template<typename T>
+struct hash_enum
+{
+  typedef size_t result_type;
+  typedef T argument_type;
+
+  size_t operator() (T val) const noexcept
+  {
+    using underlying = typename std::underlying_type<T>::type;
+    return std::hash<underlying> () (static_cast<underlying> (val));
+  }
+};
+
+} /* namespace gdb */
+
+#endif /* COMMON_HASH_ENUM_H */