Update Gentoo Linux setup to include clang
<!-- Please describe your changes on the following line: -->While other Linux distributions include clang package as required in the setup section of README, Gentoo does not. This patch fixes it.
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- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).
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gootorov commentedJan 9, 2018
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edited
./mach build -ddoes not report any errors./mach test-tidydoes not report any errorsThis change is