[llvm][cmake] Fix add_subdirectory build in multi-config

Using CMAKE_CFG_INTDIR in paths that are used in configure_file,
resulted in a folder that is literally called '${CONFIGURATION}'
for the multi-config ninja build.

I think this is a regression from a while ago. Fix this by replacing
CMAKE_CFG_INTDIR with '.'. We can only create one of the
LLVMConfig.cmake files as the consuming CMake project can only import a
single file. This creates LLVMConfig.cmake and others in the place where
they were previously and where they are for a single-config build.

Differential Revision: https://reviews.llvm.org/D139623