libunwind uses C-style and low-level C++, so the language standard
doesn't matter that much, but bumping to C++17 aligns with the rest of
LLVM and enables some features that would cause pedantic warnings in
C++11 (e.g. -Wc++17-attribute-extensions for [[fallthrough]]/
[[nodiscard]]/[[maybe_unused]]). (Contributors might use these features
unaware of the pedantic warnings).
Suggested-by: Christopher Di Bella <cjdb@google.com>
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