[Modules] Improve fixit for framework private module maps

The wrong source range was being provided in some case, fix that to get
a better fixit.

rdar://problem/38520199

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@328857 91177308-0d34-0410-b5e6-96231b3b80d8