[modules] When constructing paths relative to a module, strip out /./ directory
components. These sometimes get synthetically added, and we don't want -Ifoo
and -I./foo to be treated fundamentally differently here.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224055 91177308-0d34-0410-b5e6-96231b3b80d8