[gdb] Fix build error in macroexp.c

[gdb] Fix build error in macroexp.c

When doing a combined build with the gcc and binutils-gdb repos, I run into
this build error in gdb:
...
gdb/macroexp.c: \
  In function ‘void get_next_token_for_substitution(macro_buffer*, \
  macro_buffer*, char**, macro_buffer*, char**, int*, bool*)’:
gdb/macroexp.c:925:17: error: \
  implicitly-declared ‘constexpr macro_buffer& \
  macro_buffer::operator=(const macro_buffer&)’ is deprecated \
  [-Werror=deprecated-copy]
       *token = *lookahead;
...

Wdeprecated-copy is a new gcc warning added after gcc 8.

This patch fixes the build error by adding an explicit copy operator to the
macro_buffer class.  I've added asserts to ensure that both the dest and src
of the copy are shared, in other words, neither is owner of the text pointer.

I've run the gdb testsuite on x86_64-linux and the asserts did not trigger.

2018-07-05  Tom de Vries  <tdevries@suse.de>

	* macroexp.c (macro_buffer) <operator=>: New member function.