Move dwarf2_get_die_type declaration to dwarf2/read.h

Since its implementation is in dwarf2/read.c, its declaration belongs in
dwarf2/read.h.  Move the documentation to the .h at the same time.

gdb/ChangeLog:

	* loc.h (dwarf2_get_die_type): Move to...
	* read.h (dwarf2_get_die_type): ... here.
	* read.c (dwarf2_get_die_type): Move doc to header.