[dsymutil] Perform analyzeContextInfo and CloneDIEs in parallel

This patch makes dsymutil perform analyzeContextInfo and CloneDIEs in
parallel. For the same object file, there is a dependency between the
two. However, we can do analyzeContextInfo for the next object file
while cloning DIEs for the current. This is exactly the approach taken
in this patch.

For WebCore, this leads to a performance improvement of 29% and for
clang we see similar results with at 32% improvement.

A big thanks to Pete Cooper who came up with the original idea and
the PoC.

Differential revision: https://reviews.llvm.org/D43945

llvm-svn: 327399

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