[dsymutil] Remove recursion from lookForChildDIEsToKeep (2/2) (NFC)

The functions lookForDIEsToKeep and keepDIEAndDependencies are mutually
recursive and can cause a stackoverflow for large projects. While this
has always been the case, it became a bigger issue when we parallelized
dsymutil, because threads get only a fraction of the stack space.

This patch removes the final recursive call from lookForDIEsToKeep. The
call was used to look at the current DIE's parent chain and mark
everything as kept.

This was tested by running dsymutil on clang built in debug (both with
and without modules) and comparing the MD5 hash of the generated dSYM
companion file.

Differential revision: https://reviews.llvm.org/D70994