Add gdb::hash_enum

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.