Fix build after r206338

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206341 91177308-0d34-0410-b5e6-96231b3b80d8
Fixed problem with exclude header. The exclude header argument needs to be relative to the module.map file.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206342 91177308-0d34-0410-b5e6-96231b3b80d8