VS: Tell VS 16.4 not to verify SYMBOLIC custom command outputs

VS 16.4 introduces an additional check on `CustomBuild` rules that warns
if the outputs of the command are not created.  However, CMake supports
marking outputs with the `SYMBOLIC` property to indicate that they will
not actually be generated.  That property is used by Makefile and Ninja
generators but has not been needed by the VS generators before.  Teach
the VS generator to disable `VerifyInputsAndOutputsExist` in custom
build rules that have a symbolic output.

Fixes: #19737