[lsan] fix a rare lsan false positive: ensure that we don't re-sort the...

[lsan] fix a rare lsan false positive: ensure that we don't re-sort the chunks_ array while iterating over it. A test is hard to create, but I've added a consistency check that fires w/o the fix on existing tests. The bug analysis and the initial patch were provided by Pierre Bourdon

llvm-svn: 286478