Accelerate lookup_symbol_aux_objfile 85x

Accelerate lookup_symbol_aux_objfile 85x

During debugging I get 10-30 seconds for a response to simple commands like:
	(gdb) print vectorvar.size()
With this patch the performance gets to 1-2 seconds which is somehow
acceptable.  The problem is that dwarf2_gdb_index_functions.lookup_symbol
(quick_symbol_functions::lookup_symbol) may return (and returns) NULL even for
symbols which are present in .gdb_index but which can be found in already
expanded symtab.  But searching in the already expanded symtabs is just too
slow when there are 400000+ expanded symtabs.  There would be needed some
single global hash table for each objfile so that one does not have to iterate
all symtabs.  Which .gdb_index could perfectly serve for, just its
lookup_symbol() would need to return authoritative yes/no answers.

Even after such fix these two simple patches are useful for example for
non-.gdb_index files.

One can reproduce the slugging interactive GDB performance with:
	#include <string>
	using namespace std;
	string var;
	class C {
	public:
	  void m() {}
	};
	int main() {
	  C c;
	  c.m();
	  return 0;
	}
g++ -o slow slow.C -Wall -g $(pkg-config --libs gtkmm-3.0)
gdb ./slow -ex 'b C::m' -ex 'maintenance set per-command space' -ex 'maintenance set per-command symtab' -ex 'maintenance set per-command
time' -ex r
[...]
(gdb) p <tab><tab>
Display all 183904 possibilities? (y or n) n
(gdb) p/r var
$1 = {static npos = <optimized out>, _M_dataplus = {<std::allocator<char>> = {<__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No
data fields>}, _M_p = 0x3a4db073d8 <std::string::_Rep::_S_empty_rep_storage+24> ""}}
Command execution time: 20.023000 (cpu), 20.118665 (wall)
                        ^^^^^^^^^
Space used: 927997952 (+0 for this command)

Without DWZ there are X global blocks for X primary symtabs for X CUs of
objfile.  With DWZ there are X+Y global blocks for X+Y primary symtabs for
X+Y CUs where Y are 'DW_TAG_partial_unit's.

For 'DW_TAG_partial_unit's (Ys) their blockvector is usually empty.  But not
always, I have found there typedef symbols, there can IMO be optimized-out
static variables etc.

Neither of the patches should cause any visible behavior change.

gdb/ChangeLog
2014-12-04  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* block.c (block_lookup_symbol_primary): New function.
	* block.h (block_lookup_symbol_primary): New declaration.
	* symtab.c (lookup_symbol_in_objfile_symtabs): Assert BLOCK_INDEX.
	Call block_lookup_symbol_primary.