[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.

Author: ilovepi

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -1341,7 +1341,7 @@ if(LLVM_USE_RELATIVE_PATHS_IN_DEBUG_INFO)
  else()
  set(source_root "${LLVM_MAIN_SRC_DIR}")
  endif()
- file(RELATIVE_PATH relative_root "${source_root}" "${CMAKE_BINARY_DIR}")
+ file(RELATIVE_PATH relative_root "${CMAKE_BINARY_DIR}" "${source_root}")
  append_if(SUPPORTS_FDEBUG_PREFIX_MAP "-fdebug-prefix-map=${CMAKE_BINARY_DIR}=${relative_root}" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
  append_if(SUPPORTS_FDEBUG_PREFIX_MAP "-fdebug-prefix-map=${source_root}/=${LLVM_SOURCE_PREFIX}" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
  add_flag_if_supported("-no-canonical-prefixes" NO_CANONICAL_PREFIXES)
@@ -1356,7 +1356,7 @@ if(LLVM_USE_RELATIVE_PATHS_IN_FILES)
  else()
  set(source_root "${LLVM_MAIN_SRC_DIR}")
  endif()
- file(RELATIVE_PATH relative_root "${source_root}" "${CMAKE_BINARY_DIR}")
+ file(RELATIVE_PATH relative_root "${CMAKE_BINARY_DIR}" "${source_root}")
  append_if(SUPPORTS_FFILE_PREFIX_MAP "-ffile-prefix-map=${CMAKE_BINARY_DIR}=${relative_root}" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
  append_if(SUPPORTS_FFILE_PREFIX_MAP "-ffile-prefix-map=${source_root}/=${LLVM_SOURCE_PREFIX}" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
  add_flag_if_supported("-no-canonical-prefixes" NO_CANONICAL_PREFIXES)