Bug 937458, bump the required clang version in mach bootstrap, r=gps DONTBUILD

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extra : rebase_source : 423de887a8ee17846ad3f4b3a716d86b64ce6654
Bug 938950 - Don't provide full access to compartment/zone/runtime during Ion compilation, r=jandem. CLOBBER
Bug 940153 - Build dom/encoding in unified mode. r=bzbarsky
Bug 940185 - Build dom/audiochannel in unified mode. r=bzbarsky