VS: Restore toleration of target-wide -TP flag with MSVC
Since commit 3b547e2e4b (VS: Simplify logic adding source file C/C++
language flag to MSVC, 2020-05-15, v3.18.0-rc1~139^2~1) we only add a
per-source language selection flag when the source file extension does
not match the compiler's default. This approach breaks when a project
adds a target-wide `-TP` flag.
Although such projects likely did not work with non-VS generators, we
did support them before in Visual Studio generators. Add a special case
to tolerate such flags again.
Fixes: #21005
4 files changed
