Fix logic from commit 9df1f33c9a (VisualStudio: move PCH rules to
projects when possible., 2020-10-15, v3.20.0-rc1~638^2) to explicitly
disable PCH on sources that should not use the target-wide PCH rules.
Fixes: #21827
When the path to a header file is specified using a generator
expression, evaluation of the genex must produce an absolute path.
Update our documented example and add a test covering the case.
Fixes: #20617