VS: Do not apply '/external:W*' flag table mapping on VS < 16.10
Since !6171 (merged) we map flags to the ExternalWarningLevel
element. VS 16.9 does not support that element, but its cl
compiler does support the /external:W*
flags. Filter out the flag table entry on older VS versions.
Fixes: #22308 (closed)
Backport: release:HEAD^2