Poison XNEW and friends for types that should use new/delete

Poison XNEW and friends for types that should use new/delete

This patch (finally!) makes it so that trying to use XNEW with a type
that requires "new" will cause a compilation error.  The criterion I
initially used to allow a type to use XNEW (which calls malloc in the
end) was std::is_trivially_constructible, but then realized that gcc 4.8
did not have it.  Instead, I went with:

  using IsMallocatable = std::is_pod<T>;

which is just a bit more strict, which doesn't hurt.  A similar thing is
done for macros that free instead of allocated, the criterion is:

  using IsFreeable = gdb::Or<std::is_trivially_destructible<T>, std::is_void<T>>;

Trying to use XNEW on a type that requires new will result in an error
like this:

    In file included from /home/simark/src/binutils-gdb/gdb/common/common-utils.h:26:0,
                     from /home/simark/src/binutils-gdb/gdb/common/common-defs.h:78,
                     from /home/simark/src/binutils-gdb/gdb/defs.h:28,
                     from /home/simark/src/binutils-gdb/gdb/lala.c:1:
    /home/simark/src/binutils-gdb/gdb/common/poison.h: In instantiation of ‘T* xnew() [with T = bar]’:
    /home/simark/src/binutils-gdb/gdb/lala.c:13:3:   required from here
    /home/simark/src/binutils-gdb/gdb/common/poison.h:103:3: error: static assertion failed: Trying to use XNEW with a non-POD data type.  Use operator new instead.
       static_assert (IsMallocatable<T>::value, "Trying to use XNEW with a non-POD\
       ^~~~~~~~~~~~~

Generated-code-wise, it adds one more function call (xnew<T>) when using
XNEW and building with -O0, but it all goes away with optimizations
enabled.

gdb/ChangeLog:

	* common/common-utils.h: Include poison.h.
	(xfree): Remove declaration, add definition with static_assert.
	* common/common-utils.c (xfree): Remove.
	* common/poison.h (IsMallocatable): Define.
	(IsFreeable): Define.
	(free): Delete for non-freeable types.
	(xnew): New.
	(XNEW): Undef and redefine.
	(xcnew): New.
	(XCNEW): Undef and redefine.
	(xdelete): New.
	(XDELETE): Undef and redefine.
	(xnewvec): New.
	(XNEWVEC): Undef and redefine.
	(xcnewvec): New.
	(XCNEWVEC): Undef and redefine.
	(xresizevec): New.
	(XRESIZEVEC): Undef and redefine.
	(xdeletevec): New.
	(XDELETEVEC): Undef and redefine.
	(xnewvar): New.
	(XNEWVAR): Undef and redefine.
	(xcnewvar): New.
	(XCNEWVAR): Undef and redefine.
	(xresizevar): New.
	(XRESIZEVAR): Undef and redefine.