[Frontend] Add an ExternCSystem include entry group.

 - The only group where it makes sense for the "ExternC" bit is System, so this
   simplifies having to have the extra isCXXAware (or ImplicitExternC, depending
   on what code you talk to) bit caried around.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173859 91177308-0d34-0410-b5e6-96231b3b80d8