It was all done in one huge, unreadable method.
The refactoring along-side fixes the following:

- iOS tool chains were demoted to manual at start up because there
  was no tool chain factory that would auto detect them
- a QTC_CHECK failed because single Qt Versions could appear multiple
  times in the architecture->Qt version map, because each architecture
  is present in two abis (32-bit and 64-bit)

The refactoring also removes unneeded automatic conversion
from non-iOS Qt version to iOS Qt version, and removes considering GCC
toolchains (which were not handled anyhow, because only Clang
"platforms" were considered).

Change-Id: Ic9ae797646f159ed45959fc797990aa98f2136fb
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>