Create a correctly-sized demangled names hashtable

Create a correctly-sized demangled names hashtable

If we have a minsym count, we know the demangled names hashtable will
be at least that big.  So use that count to size it, so we don't
have to resize/rehash it as much.

This is a 6% improvement in minsym loading time.

2019-11-22  Christian Biesinger  <cbiesinger@google.com>

	* symtab.c (create_demangled_names_hash): Use per_bfd->
	minimal_symbol_count for computing the initial size, if greater
	than our default size.

Change-Id: I1f074d38e1d90af58705ec852f90c84cc034cd2e