Move dwarf2_read_addr_index declaration to dwarf2/read.h

The implementation is in dwarf2/read.c, so the declaration belongs in
dwarf2/read.h.  Also, move the documentation there.

gdb/ChangeLog:

	* dwarf2/loc.h (dwarf2_read_addr_index): Move...
	* dwarf2/read.h (dwarf2_read_addr_index): ... here.
	* dwarf2/read.c (dwarf2_read_addr_index): Move doc to header.