MSVC: Use 'pragma system_header' in PCH only on cl 19.13 and above
VS 2017 15.6 introduced support for this pragma. Older MSVC versions warn that it is unknown.
Fixes: #20692 (closed)
Backport: release
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VS 2017 15.6 introduced support for this pragma. Older MSVC versions warn that it is unknown.
Fixes: #20692 (closed)
Backport: release