Hide "#pragma optimize("", off)" from clang when it pretends to be MSVC 2017
Hide "#pragma optimize("", off)" from clang when it pretends to be MSVC 2017

These pragmas work around a bug in VC 1911 that isn't present in clang,
and clang warns about them.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@315699 91177308-0d34-0410-b5e6-96231b3b80d8

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