Generate a separate compile_commands.json DB for external projects

[george.karpenkov]

Summary

Compile_commands.json file is very useful both for tooling and for reproducible builds.
For files generated from recursive CMake invocation this information was not previously generated.

[george.karpenkov]

@beanz ping, should be OK to merge, right?

[beanz]

LGTM!

cmake/modules/LLVMExternalProjectUtils.cmake

CMAKE_ARGS ${${nameCanon}_CMAKE_ARGS}
                ${compiler_args}
                -DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}
                -DLLVM_BINARY_DIR=${PROJECT_BINARY_DIR}
                -DLLVM_CONFIG_PATH=$<TARGET_FILE:llvm-config>
                -DLLVM_ENABLE_WERROR=${LLVM_ENABLE_WERROR}
                -DPACKAGE_VERSION=${PACKAGE_VERSION}
                -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
                -DCMAKE_MAKE_PROGRAM=${CMAKE_MAKE_PROGRAM}
+               -DCMAKE_EXPORT_COMPILE_COMMANDS=1
                ${ARG_CMAKE_ARGS}
                ${PASSTHROUGH_VARIABLES}
     INSTALL_COMMAND ""
     STEP_TARGETS configure build
     BUILD_ALWAYS 1
     USES_TERMINAL_CONFIGURE 1
     USES_TERMINAL_BUILD 1
     USES_TERMINAL_INSTALL 1