Fix case-sensitivity of inferred framework modules

Just because we can open a directory named "COcoa.framework" doesn't
mean we should provide a "COcoa" module on a case-insensitive filesystem.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212975 91177308-0d34-0410-b5e6-96231b3b80d8