[Frontend] Factor out helper function, for clarity.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173853 91177308-0d34-0410-b5e6-96231b3b80d8
[Frontend] Rename a member variable to clarify its intent.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173854 91177308-0d34-0410-b5e6-96231b3b80d8
[Frontend] Make the include dir group independent from the "use sysroot" bit.

 - This slightly decouples the path handling, since before the group sometimes
   dominated the "use sysroot" bit, but it was still passed in via the API.

 - No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173855 91177308-0d34-0410-b5e6-96231b3b80d8