Move more declarations from dwarf2/loc.h to dwarf2/read.h

All these functions have their implementations in dwarf2/read.c, so move
their declarations to dwarf2/read.h.  Move the doc to the header, at the
same time.

gdb/ChangeLog:

	* dwarf2/loc.h (dwarf2_fetch_die_loc_sect_off,
	dwarf2_fetch_die_loc_cu_off, dwarf2_fetch_constant_bytes,
	dwarf2_fetch_die_type_sect_off): Move to...
	* dwarf2/read.h (dwarf2_fetch_die_loc_sect_off,
	dwarf2_fetch_die_loc_cu_off, dwarf2_fetch_constant_bytes,
	dwarf2_fetch_die_type_sect_off): ... here.
	* dwarf2/read.c (dwarf2_fetch_die_loc_sect_off,
	dwarf2_fetch_die_loc_cu_off, dwarf2_fetch_constant_bytes,
	dwarf2_fetch_die_type_sect_off): Move doc to header file.