[Modules] If a module map resides in a system header directory, treat it as a system module.

This prevents -pedantic from causing warnings in the system headers
used to create modules. Fixes <rdar://problem/14201171>.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184560 91177308-0d34-0410-b5e6-96231b3b80d8