[Reland][Runtimes] Merge 'compile_commands.json' files from runtimes build (#116303)

Summary:
When building a project in a runtime mode, the compilation database is a
separate CMake invocation. So its `compile_commands.json` file will be
placed elsewhere in the `runtimes/runtime-bins` directory. This is
somewhat annoying for ongoing development when a runtimes build is
necessary. This patch adds some CMake magic to merge the two files.

Fixed issue w/ standalone runtimes build by checking if the LLVM src and
CMake src are the same.

runtimes/CMakeLists.txt

@@ -346,3 +346,21 @@ if(SUB_COMPONENTS)
       ${CMAKE_CURRENT_BINARY_DIR}/runtimes/Components.cmake)
   endif()
 endif()
+
+# If the user requested 'compile_commands.json' we merge the generated JSON from
+# the created directories.
+if(CMAKE_EXPORT_COMPILE_COMMANDS AND NOT ("${LLVM_BINARY_DIR}" STREQUAL "${CMAKE_BINARY_DIR}"))
+  # Make a dependency so that we don't error if the file gets deleted somehow.
+  add_custom_command(OUTPUT ${CMAKE_BINARY_DIR}/compile_commands.json
+                     COMMAND ${CMAKE_COMMAND} -E touch ${CMAKE_BINARY_DIR}/compile_commands.json)
+
+  file(TO_NATIVE_PATH "${LLVM_MAIN_SRC_DIR}/utils/merge-json.py" MERGE_JSON_PATH)
+  add_custom_command(OUTPUT ${LLVM_BINARY_DIR}/compile_commands.json
+                     COMMAND ${CMAKE_COMMAND} -E touch ${LLVM_BINARY_DIR}/compile_commands.json
+                     COMMAND ${Python3_EXECUTABLE} ${MERGE_JSON_PATH}
+                             ${LLVM_BINARY_DIR}/compile_commands.json
+                             ${CMAKE_BINARY_DIR}/compile_commands.json
+                             -o ${LLVM_BINARY_DIR}/compile_commands.json
+                     DEPENDS ${CMAKE_BINARY_DIR}/compile_commands.json)
+  add_custom_target(merge_runtime_commands ALL DEPENDS ${LLVM_BINARY_DIR}/compile_commands.json)
+endif()