gdb: add accessors for field (and call site) location

Add accessors for the various location values in struct field.  This
lets us assert that when we get a location value of a certain kind (say,
bitpos), the field's location indeed contains a value of that kind.

Remove the SET_FIELD_* macros, instead use the new setters directly.
Update the FIELD_* macros used to access field locations to go through
the getters.  They will be removed in a subsequent patch.

There are places where the FIELD_* macros are used on call_site_target
structures, because it contains members of the same name (loc_kind and
loc).  For now, I have replicated the getters/setters in
call_site_target.  But we could perhaps eventually factor them in a
"location" structure that can be used at both places.

Note that the field structure, being zero-initialized, defaults to a
bitpos location with value 0.  While writing this patch, I tried to make
it default to an "unset" location, to catch places where we would miss
setting a field's location.  However, I found that some places relied on
the default being "bitpos 0", so I left it as-is.  This change could
always be done as follow-up work, making these places explicitly set the
"bitpos 0" location.

I found two issues to fix:

 - I got some failures in the gdb.base/infcall-nested-structs-c++.exp
   test.  They were caused by two functions in amd64-tdep.c using
   TYPE_FIELD_BITPOS before checking if the location is of the bitpos
   kind, which they do indirectly through `field_is_static`.  Simply
   move getting the bitpos below the field_is_static call.

 - I got a failure in gdb.xml/tdesc-regs.exp.  It turns out that in
   make_gdb_type_enum, we set enum field values using SET_FIELD_BITPOS,
   and later access them through FIELD_ENUMVAL.  Fix that by using
   set_loc_enumval to set the value.

Change-Id: I53d3734916c46457576ba11dd77df4049d2fc1e8

gdb/rust-lang.c

@@ -912,7 +912,7 @@ rust_composite_type (struct type *original,
     {
       struct field *field = &result->field (i);
 
-      SET_FIELD_BITPOS (*field, bitpos);
+      field->set_loc_bitpos (bitpos);
       bitpos += TYPE_LENGTH (type1) * TARGET_CHAR_BIT;
 
       field->set_name (field1);
@@ -933,7 +933,7 @@ rust_composite_type (struct type *original,
 	  if (delta != 0)
 	    bitpos += align - delta;
 	}
-      SET_FIELD_BITPOS (*field, bitpos);
+      field->set_loc_bitpos (bitpos);
 
       field->set_name (field2);
       field->set_type (type2);