Refactoring in commit 3b547e2e (VS: Simplify logic adding source file
C/C++ language flag to MSVC, 2020-05-15, v3.18.0-rc1~139^2~1) failed to
account for MSVC's treatment of `.C` extensions as C.  Add this special
case to the logic to restore use of `-TP` for `.C` extensions.

Fixes: #20822