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This CL implements the Clang code coverage compiler wrapper, which can
be used to instrument a subset of source files, and the main use case
is to enable generating code coverage reports at per-cl level during
try jobs.

This CL only focuses on the wrapper itself, how to hook this wrapper
into the build system will be addressed in a separate CL.

Bug: 898695
Change-Id: Idb21640b5566ce78089059b3d0116390b488a383
Reviewed-on: https://chromium-review.googlesource.com/c/1301969
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604381}