[gdb/symtab] Do more zero-initialization of type::fields

Now that we've introduced type::{alloc_fields,copy_fields}, the places where
no zero-initialization of allocated fields is done are easy to spot:
...
$ find gdb* -type f | grep -v ChangeLog | xargs grep alloc_fields | grep false
gdb/coffread.c:  type->alloc_fields (nfields, false);
gdb/coffread.c:  type->alloc_fields (nsyms, false);
gdb/stabsread.c:	  ftype->alloc_fields (nsemi, false);
gdb/gdbtypes.c:  resolved_type->alloc_fields (nfields, false);
gdb/gdbtypes.c:  alloc_fields (nfields, false);
gdb/gdbtypes.c:  alloc_fields (nfields, false);
gdb/mdebugread.c:	t->alloc_fields (nfields, false);
gdb/mdebugread.c:		  ftype->alloc_fields (nparams, false);
...

All hits in gdbtypes.c are ok.  There are two hits in the two variants of
copy_fields, and there's already a comment for the third.

AFAICT, the other ones are not ok, so fix those by dropping the "false"
argument.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>