Add an optional separate install prefix for internal components. rdar://10217046

Some files installed by clang are not relevant for general users and we'd like
to be able to install them to a different location.  This adds a new
--with-internal-prefix configure option and a corresponding PROJ_internal_prefix
makefile variable, which defaults to the standard prefix.  A tool makefile
can specify that it should be installed to this internal prefix by defining
INTERNAL_TOOL.

llvm-svn: 145234