[cmake] Fix relative paths in prefix map (#74132)

When building debug version of LLVM with
`LLVM_USE_RELATIVE_PATHS_IN_FILES=On` would cause source paths to be
incorrect, and be prefixed by the build directory. This lead to source
locations like the following: `../build/llvm/...`. Such paths do not
exist, and existing debuggers can't adjust their search location because
of the incorrect prefix. Ultimately, this happened because the relative
path creation goes in the wrong direction, from source-dir to build-dir
instead of from build-dir to source-dir.

This patch swaps the directionality of the relative paths so that they
get a proper prefix from the build directory. Given a build dir at
`/build` and a project directory at `/llvm-project`, we get source
locations like: `../llvm-project/llvm/lib/Transforms/...`, which a
debugger can resolve once pointed to the correct project directory.