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The code coverage reports from llvm-cov will emit detailed coverage
information for instantiated templates, but by default, will emit
the method names using their mangled form. llvm-cov allows for
providing a custom demangler.

This conditionally provides custom demangler, using c++filt when
running on Linux and macOS. Longer-term, a solution based on
llvm-symbolizer is more desirable for platform neutrality, but
this helps bridge the short-term need.

BUG=843246

Change-Id: I718016b05af0842ac148d06e968f362bc68b5fc8
Reviewed-on: https://chromium-review.googlesource.com/1060264
Commit-Queue: Ryan Sleevi <rsleevi@chromium.org>
Reviewed-by: Abhishek Arya <inferno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558863}