[modules] Fix "prefer own module over others" rule when selecting a module for...

[modules] Fix "prefer own module over others" rule when selecting a module for a header to work in the presence of module hierarchy.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@241936 91177308-0d34-0410-b5e6-96231b3b80d8