[dsymutil] Prevent non-determinism due to threading.

Before this patch, analyzeContext called getCanonicalDIEOffset(), for
which the result depends on the timings of the setCanonicalDIEOffset()
calls in the cloneLambda. This can lead to slightly different output
between runs due to threading.

To prevent this from happening, we now record the output debug info size
after importing the modules (before any concurrent processing takes
place). This value, named the ModulesEndOffset is used to compare the
canonical DIE offset against. If the value is greater than this offset,
the canonical DIE offset has been updated during cloning, and should
therefore not be considered for pruning.

Differential revision: https://reviews.llvm.org/D51443

llvm-svn: 341649