Teach LLVMConfig to avoid modifying CMAKE_MODULE_PATH

Do not modify this value on the application's behalf and just ensure API
modules are always available next to the LLVMConfig module.  This is
already the case in the install tree so use file(COPY) to make it so in
the build tree.  Include the LLVM-Config API module from next to the
LLVMConfig location.

Contributed by Brad King.

llvm-svn: 201047