gdb: move call site types to call-site.h

gdb: move call site types to call-site.h

I hesitated between putting  the file in the dwarf2 directory (as
gdb/dwarf2/call-site.h) or in the common directory (as gdb/call-site.h).
The concept of call site is not DWARF-specific, another debug info
reader could provide this information.  But as it is, the implementation
is a bit DWARF-specific, as one form it can take is a DWARF expression
and parameters can be defined using a DWARF register number.  So I ended up
choosing to put it under dwarf2/.  If another debug info reader ever
wants to provide call site information, we can introduce a layer of
abstraction between the "common" call site and the "dwarf2" call site.

The copyright start year comes from the date `struct call_site` was
introduced.

Change-Id: I1cd84aa581fbbf729edc91b20f7d7a6e0377014d
Reviewed-By: Bruno Larsen <blarsen@redhat.com>