Remove tui_alloc_source_buffer

Remove tui_alloc_source_buffer

There is no longer any need for tui_alloc_source_buffer.  The two
callers of this function immediately change the contents of the
window, undoing the work done by this function.

This required adding a move constructor to tui_source_element -- a
mildly surprising find, but without this, resizing the vector will
cause crashes.  This issue was masked earlier because
tui_alloc_source_buffer handled this.

Note that a patch for this bug was submitted here:

    https://sourceware.org/ml/gdb-patches/2019-08/msg00094.html

That patch is better, IMO, but the author as yet hasn't responded to a
request for a ChangeLog entry.

gdb/ChangeLog
2019-08-15  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (tui_alloc_source_buffer): Don't declare.
	(struct tui_source_element): Add DISABLE_COPY_AND_ASSIGN, and move
	constructor.
	* tui/tui-winsource.c (tui_alloc_source_buffer): Remove.
	* tui/tui-source.c (tui_set_source_content): Update.
	* tui/tui-disasm.c (tui_set_disassem_content): Update.