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[clang-doc] Implement reducer portion of the frontend framework

Description

[clang-doc] Implement reducer portion of the frontend framework

Implements a simple, in-memory reducer for the mapped output of the
initial tool. This creates a collection object for storing the
deduplicated infos on each declaration, and populates that from the
mapper output. The collection object is serialized to LLVM
bitstream. On reading each serialized output, it checks to see if a
merge is necessary and if so, merges the new info with the existing
info (prefering the existing one if conflicts exist).

For a more detailed overview of the tool, see the design document
on the mailing list:
http://lists.llvm.org/pipermail/cfe-dev/2017-December/056203.html

Differential Revision: https://reviews.llvm.org/D43341