[libclang] Introduce clang_VirtualFileOverlay_setCaseSensitivity that exposes the VFS option
to set the case-sensitivity for lookups.

rdar://16374696

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204303 91177308-0d34-0410-b5e6-96231b3b80d8