gdb: add block ordering predicate for ordering blocks in blockvector

gdb: add block ordering predicate for ordering blocks in blockvector

This commit adds blockvector::block_less_than() predicate that defines
required ordering of blocks within blockvector.

It orders blocks so that blocks with lower start address come before
blocks with higher start address.  If two blocks start at the same
address, enclosing (larger) block should come before nested (smaller)
block.

This ordering is depended upon in find_block_in_blockvector(). Although
its comment did not say so, find_block_in_blockvector() is called from
blockvector_for_pc_sect() which is explicit about it. While at it, I
changed the comment of find_block_in_blockvector() to say so explicitly
too.

As Andrew pointed out, buildsym.c sorts block slightly differently,
taking only the start address into account.  The comment there says
blocks with same start address should not be reordered as they are in
correct order already and that order is needed.  It is unclear to me
if buildsym.c arranges blocks starting at the same address in required
order before sorting them or this happens "by chance".  I did modify
buildsym_compunit::make_blockvector() to assert blocks are properly
ordered and running testsuite did not show any regressions.

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