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 (closed)