Fix GDB build with G++ 4.8

Fix GDB build with G++ 4.8

G++ 4.8 trips on:

  In file included from /opt/gcc-4.8/include/c++/4.8.5/algorithm:62:0,
		   from ../../src/gdb/ada-lang.c:65:
  /opt/gcc-4.8/include/c++/4.8.5/bits/stl_algo.h: In instantiation of ‘_RandomAccessIterator std::__unguarded_partition(_RandomAccessIterator, _RandomAccessIterator, const _Tp&) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<ada_exc_info*, std::vector<ada_exc_info> >; _Tp = ada_exc_info]’:
  /opt/gcc-4.8/include/c++/4.8.5/bits/stl_algo.h:2283:70:   required from ‘_RandomAccessIterator std::__unguarded_partition_pivot(_RandomAccessIterator, _RandomAccessIterator) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<ada_exc_info*, std::vector<ada_exc_info> >]’
  /opt/gcc-4.8/include/c++/4.8.5/bits/stl_algo.h:2315:54:   required from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<ada_exc_info*, std::vector<ada_exc_info> >; _Size = long int]’
  /opt/gcc-4.8/include/c++/4.8.5/bits/stl_algo.h:5461:36:   required from ‘void std::sort(_RAIter, _RAIter) [with _RAIter = __gnu_cxx::__normal_iterator<ada_exc_info*, std::vector<ada_exc_info> >]’
  ../../src/gdb/ada-lang.c:13153:61:   required from here
  /opt/gcc-4.8/include/c++/4.8.5/bits/stl_algo.h:2245:19: error: passing ‘const ada_exc_info’ as ‘this’ argument of ‘bool ada_exc_info::operator<(const ada_exc_info&)’ discards qualifiers [-fpermissive]
      while (__pivot < *__last)
		     ^

Seems to be a libstdc++ bug meanwhile fixed by:
  https://gcc.gnu.org/ml/libstdc++/2012-04/msg00074.

In any case, there's no reason these methods can't be const.

gdb/ChangeLog:
2017-10-02  Tom Tromey  <tom@tromey.com>
	    Pedro Alves  <palves@redhat.com>

	* ada-lang.h (ada_exc_info::operator<): Make const.
	(ada_exc_info::operator==): Make const.
	* ada-lang.c (ada_exc_info::operator<, ada_exc_info::operator==):
	Make const.