Bug#952718: qtcreator: Clang code model fail to find stddef.h if libclang-common-8-dev package is not installed

[Oswald Buddenhagen <oswald.buddenhagen@gmx.de>]

Package: qtcreator
Version: 4.11.0-2
Followup-For: Bug #952718

i just wasted about half an hour on researching this, so i'll summarize 
my current understanding:

libclang-N needs to depend on libclang-common-N-dev, as otherwise it's 
dysfunctional.

failing that, qtcreator needs to recommend libclang-common-N-dev. this 
could be upgraded to depends if the clang-related plugins incl.  
libclangsupport are factored out to a separate qtcreator sub-package 
which qtcreator recommends.
alternatively, this approach could be used to isolate the libclang-N 
dependency itself.

that qtcreator's code model technically uses the wrong headers is 
correct, but orthogonal to the issue, and usually not problematic (which 
is why the approach was deemed accepatable in the first place).