.gdb_index prod perf regression: mapped_symtab now vector of values

.gdb_index prod perf regression: mapped_symtab now vector of values

... instead of vector of pointers

There's no real reason for having mapped_symtab::data be a vector of
heap-allocated symtab_index_entries.  symtab_index_entries is not that
large, it's movable, and it's cheap to move.  Making the vector hold
values instead improves cache locality and eliminates many roundtrips
to the heap.

Using the same test as in the previous patch, against the same gdb
inferior, timing improves ~13% further:

  ~6.0s => ~5.2s (average of 5 runs).

Note that before the .gdb_index C++ifycation patch, we were at ~5.7s.
We're now consistenly better than before.

gdb/ChangeLog
2017-06-12  Pedro Alves  <palves@redhat.com>

	* dwarf2read.c (mapped_symtab::data): Now a vector of
	symtab_index_entry instead of vector of
	std::unique_ptr<symtab_index_entry>.  All users adjusted to check
	whether an element's name is NULL instead of checking whether the
	element itself is NULL.
	(find_slot): Change return type.  Adjust.
	(hash_expand, , add_index_entry, uniquify_cu_indices)
	(write_hash_table): Adjust.