Introduce buildsym-legacy.h

Introduce buildsym-legacy.h

This introduces a new header, buildsym-legacy.h, and changes all the
symbol readers to use it.  The idea is to put the function-based
interface, that relies on the buildsym_compunit global, into a
separate header.  Then when a symbol reader is updated to use the new
interface, it can simply not include buildsym-legacy.h, so it's easy
to be sure that the new API is used everywhere.

gdb/ChangeLog
2018-07-20  Tom Tromey  <tom@tromey.com>

	* xcoffread.c: Include buildsym-legacy.h.
	* windows-nat.c: Include buildsym-legacy.h.
	* stabsread.c: Include buildsym-legacy.h.
	* mdebugread.c: Include buildsym-legacy.h.
	* buildsym-legacy.h: New file.
	* buildsym-legacy.c: New file, from buildsym.c.
	* go32-nat.c: Include buildsym-legacy.h.
	* dwarf2read.c: Include buildsym-legacy.h.
	* dbxread.c: Include buildsym-legacy.h.
	* cp-namespace.c: Include buildsym-legacy.h.
	* coffread.c: Include buildsym-legacy.h.
	* buildsym.h: Move some contents to buildsym-legacy.h.
	* buildsym.c: Include buildsym-legacy.h.  Move many functions to
	buildsym-legacy.c.
	* Makefile.in (HFILES_NO_SRCDIR): Add buildsym-legacy.h.