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[clang-scan-deps] initial outline of the tool that runs preprocessor to find

Description

[clang-scan-deps] initial outline of the tool that runs preprocessor to find
dependencies over a JSON compilation database

This commit introduces an outline for the clang-scan-deps tool that will be
used to implement fast dependency discovery phase using implicit modules for
explicit module builds.

The initial version of the tool works by computing non-modular header dependencies
for files in the compilation database without any optimizations
(i.e. without source minimization from r362459).
The tool spawns a number of worker threads to run the clang compiler workers in parallel.

The immediate goal for clang-scan-deps is to create a ClangScanDeps library
which will be used to build up this tool to use the source minimization and
caching multi-threaded filesystem to implement the optimized non-incremental
dependency scanning phase for a non-modular build. This will allow us to do
benchmarks and comparisons for performance that the minimization and caching give us

Differential Revision: https://reviews.llvm.org/D60233