[CMake] Make the CMake files (LLVMConfig.cmake and LLVMExports.cmake)

[CMake] Make the CMake files (LLVMConfig.cmake and LLVMExports.cmake)
generated by the Autoconf/Makefile build system relocatable.

Previously the generated CMake files contained hardcoded paths which
prevented a binary installation from being relocated to a different
place in the file system. This problem was most noticeable in LLVM's
official binary releases which were completely unusable by a downstream
project trying to import the CMake targets.

Package maintainers who choose to modify the install location of the
CMake directory without using the ``PROJ_cmake`` Makefile variable
override will need to patch the generated``LLVMConfig.cmake`` so that
``LLVM_INSTALL_PREFIX`` and ``_LLVM_CMAKE_DIR`` variables are set
correctly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241080 91177308-0d34-0410-b5e6-96231b3b80d8