VS: Simplify generation of per-source PCH settings

Avoid looking up the PCH create/use flags just to map them through
flag tables back to the `.vcxproj` settings.  Instead, simply
generate the PCH settings directly for each source file.

Since commit 9df1f33c9a (VisualStudio: move PCH rules to projects when
possible., 2020-10-15, v3.20.0-rc1~638^2) we already do this for the
target-wide PCH settings.