[dsymutil] Convert recursion in lookForDIEsToKeep into worklist.

The functions `lookForDIEsToKeep` and `keepDIEAndDependencies` can have
some very deep recursion. This tackles part of this problem by removing
the recursion from `lookForDIEsToKeep` by turning it into a worklist.

The difficulty in doing so is the computation of incompleteness, which
depends on the incompleteness of its children. To compute this, we
insert "continuation markers" into the worklist. This informs the work
loop to (re)compute the incompleteness property of the DIE associated
with it (i.e. the parent of the previously processed DIE).

This patch should generate byte-identical output. Unfortunately it also
has some impact of performance, regressing by about 4% when processing
clang on my machine.

Differential revision: https://reviews.llvm.org/D48899

llvm-svn: 338536