[runtimes] Allow use of external llvm-lit on standalone builds

[pratlucas]

When creating a standalone build of the runtimes sub-project, the
current CMake implementation looks for a lit executable that might
potentially exist in the build tree and unconditionally overrides the
value of LLVM_EXTERNAL_LIT. Due to this, any value passed via
-DLLVM_EXTERNAL_LIT when configuring the CMake project is ignored.
This change adds the ALLOW_EXTERNAL argument to the
get_llvm_lit_path call in the runtimes' CMakeLists.txt, allowing any
value previously set to be considered.

[arichardson]

This looks fine to me, although the setting of LLVM_EXTERNAL_LIT_MISSING_WARNED_ONCE is incorrect if LLVM_EXTERNAL_LIT was already set when entering this file. Probably doesn't matter since people who manually set LLVM_EXTERNAL_LIT probably know what they are doing and if it is set to something invalid, then we just fail a bit later in the configure process instead.

[pratlucas]

Thanks for the review!

When ALLOW_EXTERNAL is used, the get_llvm_lit_path function itself will check if the path provided exists and issue the warning, so I believe it's still ok the set LLVM_EXTERNAL_LIT_MISSING_WARNED_ONCE after that call:

llvm-project/llvm/cmake/modules/AddLLVM.cmake

Lines 2039 to 2050 in 64bd485

	if (NOT "${LLVM_EXTERNAL_LIT}" STREQUAL "")
	if (EXISTS ${LLVM_EXTERNAL_LIT})
	get_filename_component(LIT_FILE_NAME ${LLVM_EXTERNAL_LIT} NAME)
	get_filename_component(LIT_BASE_DIR ${LLVM_EXTERNAL_LIT} DIRECTORY)
	set(${file_name} ${LIT_FILE_NAME} PARENT_SCOPE)
	set(${base_dir} ${LIT_BASE_DIR} PARENT_SCOPE)
	return()
	elseif (NOT DEFINED CACHE{LLVM_EXTERNAL_LIT_MISSING_WARNED_ONCE})
	message(WARNING "LLVM_EXTERNAL_LIT set to ${LLVM_EXTERNAL_LIT}, but the path does not exist.")
	set(LLVM_EXTERNAL_LIT_MISSING_WARNED_ONCE YES CACHE INTERNAL "")
	endif()
	endif()

[arichardson]

Thanks for the review!

When ALLOW_EXTERNAL is used, the get_llvm_lit_path function itself will check if the path provided exists and issue the warning, so I believe it's still ok the set LLVM_EXTERNAL_LIT_MISSING_WARNED_ONCE after that call:

llvm-project/llvm/cmake/modules/AddLLVM.cmake

Lines 2039 to 2050 in 64bd485

	if (NOT "${LLVM_EXTERNAL_LIT}" STREQUAL "")
	if (EXISTS ${LLVM_EXTERNAL_LIT})
	get_filename_component(LIT_FILE_NAME ${LLVM_EXTERNAL_LIT} NAME)
	get_filename_component(LIT_BASE_DIR ${LLVM_EXTERNAL_LIT} DIRECTORY)
	set(${file_name} ${LIT_FILE_NAME} PARENT_SCOPE)
	set(${base_dir} ${LIT_BASE_DIR} PARENT_SCOPE)
	return()
	elseif (NOT DEFINED CACHE{LLVM_EXTERNAL_LIT_MISSING_WARNED_ONCE})
	message(WARNING "LLVM_EXTERNAL_LIT set to ${LLVM_EXTERNAL_LIT}, but the path does not exist.")
	set(LLVM_EXTERNAL_LIT_MISSING_WARNED_ONCE YES CACHE INTERNAL "")
	endif()
	endif()

Ah yes, sounds good to me then :)