qdoc: Create and Visit the PCH always

The PCH for module QtPlatformHeaders was not being built
because there are no .cpp files there, and clangqdoc was
only building the PCH when it was asked to parse the first
.cpp file. Now it builds the PCH by a direct call from the
main.cpp file, after the headers have been gathered into a
list, but before the .cpp files are parsed.

This does reduce the qdoc error count by a few, but it does
not fix the documentation for QtPlatformHeaders. From my
debugging, it appears that visiting the PCH produces no qdoc
nodes for the C++ classes and functions, etc in the PCH.

This update also improves the selection of default include
paths when inadequate include paths are provided. That is
the case for module QtPlatformHeaders, where no include
paths are provided. The algorithm for constructing a list
of reasonable guesses is modified here.

Change-Id: I0dee8dc0279894a4482df30703657558cdb098de
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>