gdb: add gdb::make_unique function

gdb: add gdb::make_unique function

While GDB is still C++11, lets add a gdb::make_unique template
function that can be used to create std::unique_ptr objects, just like
the C++14 std::make_unique.

If GDB is being compiled with a C++14 compiler then the new
gdb::make_unique function will delegate to the std::make_unique.  I
checked with gcc, and at -O1 and above gdb::make_unique will be
optimised away completely in this case.

If C++14 (or later) becomes our minimum, then it will be easy enough
to go through the code and replace gdb::make_unique with
std::make_unique later on.

I've make use of this function in all the places I think this can
easily be used, though I'm sure I've probably missed some.

Should be no user visible changes after this commit.

Approved-By: Tom Tromey <tom@tromey.com>