Got PCH support working for MSVC.

But ... it requires sharing the same PDB files for all
compilation units that use the PCH.
For now, PCH support is still officially disabled for MSVC.
Enabled PCH support for MSVC.

PCH support for MSVC is now mostly on par with GCC support, in
terms of flexible creation and usage from any location.
The rationale is now documented as well.

Removed "minimalRebuild" support from MSVC options, because
it's at odds with PCH creation using /Z7.