Eliminate the two-level data structures behind location_specs

Currently, there's the location_spec hierarchy, and then some
location_spec subclasses have their own struct type holding all their
data fields.

I.e., there is this:

 location_spec
   explicit_location_spec
   linespec_location_spec
   address_location_spec
   probe_location_spec

and then these separate types:

  explicit_location
  linespec_location

where:

  explicit_location_spec
     has-a explicit_location
  linespec_location_spec
     has-a linespec_location

This patch eliminates explicit_location and linespec_location,
inlining their members in the corresponding location_spec type.

The location_spec subclasses were the ones currently defined in
location.c, so they are moved to the header.  Since the definitions of
the classes are now visible, we no longer need location_spec_deleter.

Some constructors that are used for cloning location_specs, like:

  explicit explicit_location_spec (const struct explicit_location *loc)

... were converted to proper copy ctors.

In the process, initialize_explicit_location is eliminated, and some
functions that returned the "data type behind a locspec", like
get_linespec_location are converted to downcast functions, like
as_linespec_location_spec.

Change-Id: Ia31ccef9382b25a52b00fa878c8df9b8cf2a6c5a

gdb/break-catch-throw.c

@@ -225,12 +225,9 @@ exception_catchpoint::re_set ()
 	 catchpoint mode.  */
       try
 	{
-	  struct explicit_location explicit_loc;
-
-	  initialize_explicit_location (&explicit_loc);
-	  explicit_loc.function_name
-	    = ASTRDUP (exception_functions[kind].function);
-	  location_spec_up locspec = new_explicit_location_spec (&explicit_loc);
+	  location_spec_up locspec
+	    = (new_explicit_location_spec_function
+	       (exception_functions[kind].function));
 	  sals = this->decode_location_spec (locspec.get (), filter_pspace);
 	}
       catch (const gdb_exception_error &ex)