Josh Stone 2304c25f31 Update the minimum external LLVM to 7
LLVM 7 is over a year old, which should be plenty for compatibility. The
last LLVM 6 holdout was llvm-emscripten, which went away in #65501.

I've also included a fix for LLVM 8 lacking `MemorySanitizerOptions`,
which was broken by #66522.
2019-12-02 11:36:21 -08:00
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2019-12-02 11:36:21 -08:00