Install c-index-test and clang-c/Index.h as internal files. rdar://10217046

Specify that these files should be installed to the optional internal
install location as specified by configure's --with-internal-prefix.
If that option is not used, they'll be installed to the default prefix
as before.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@145235 91177308-0d34-0410-b5e6-96231b3b80d8