Fix Modules/{builtin-import.mm,umbrella-header-include-builtin.mm} to be able...

Fix Modules/{builtin-import.mm,umbrella-header-include-builtin.mm} to be able to handle non-Darwin targets

Summary: Also makes them pass on Darwin, if the default target triple is a Linux triple.

Reviewers: bruno, rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D38364

llvm-svn: 314524