The mapping for this flag was added by commit 43aa632f (VS: Populate
`-Qspectre-` flag table entry for v142, 2019-01-24, v3.14.0-rc1~74^2~7).
However, it did not do anything because the special logic added by
commit bb60ed6e (VS: Add flag table entry for -Qspectre, 2018-10-08,
v3.13.0-rc1~4^2) to move the `SpectreMitigation` element from
`ClCompile` to the top level only handled the presence of the setting
and not its value.  Extend the special logic to carry the value too.

Fixes: #19535