[cmake] Fix get_llvm_lit_path() to respect LLVM_EXTERNAL_LIT always

[mgorny]

Summary

Refactor the get_llvm_lit_path() logic to respect LLVM_EXTERNAL_LIT,
and require the fallback to be defined explicitly
as LLVM_DEFAULT_EXTERNAL_LIT. This fixes building libcxx standalone
after r346888.

The old logic was using LLVM_EXTERNAL_LIT both as user-defined cache
variable and an optional pre-definition of default value from caller
(e.g. libcxx). It included a hack to make this work by assigning
the value back and forth but it was fragile and stopped working
in libcxx.

The new logic is simpler and more transparent. Default value is
provided in a separate variable, and used only when user-specified
variable is empty (i.e. not overriden).

[sgraenitz]

LGTM

libcxx/cmake/Modules/HandleOutOfTreeLLVM.cmake

macro(configure_out_of_tree_llvm)
   # llvm-lit script for us.  So we need to provide an explicit directory that
   # the configurator should write the script into.
   set(LLVM_LIT_OUTPUT_DIR "${libcxx_BINARY_DIR}/bin")
 
   if (LLVM_INCLUDE_TESTS)
     # Required LIT Configuration ------------------------------------------------
     # Define the default arguments to use with 'lit', and an option for the user
     # to override.
-    set(LLVM_EXTERNAL_LIT "${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py")
+    set(LLVM_DEFAULT_EXTERNAL_LIT "${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py")
     set(LIT_ARGS_DEFAULT "-sv --show-xfail --show-unsupported")
     if (MSVC OR XCODE)
       set(LIT_ARGS_DEFAULT "${LIT_ARGS_DEFAULT} --no-progress-bar")
     endif()
     set(LLVM_LIT_ARGS "${LIT_ARGS_DEFAULT}" CACHE STRING "Default options for lit")
   endif()
 
   # Required doc configuration

libcxxabi/cmake/Modules/HandleOutOfTreeLLVM.cmake

macro(configure_out_of_tree_llvm)
   # llvm-lit script for us.  So we need to provide an explicit directory that
   # the configurator should write the script into.
   set(LLVM_LIT_OUTPUT_DIR "${libcxxabi_BINARY_DIR}/bin")
 
   if (LLVM_INCLUDE_TESTS)
     # Required LIT Configuration ------------------------------------------------
     # Define the default arguments to use with 'lit', and an option for the user
     # to override.
-    set(LLVM_EXTERNAL_LIT "${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py")
+    set(LLVM_DEFAULT_EXTERNAL_LIT "${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py")
     set(LIT_ARGS_DEFAULT "-sv --show-xfail --show-unsupported")
     if (MSVC OR XCODE)
       set(LIT_ARGS_DEFAULT "${LIT_ARGS_DEFAULT} --no-progress-bar")
     endif()
     set(LLVM_LIT_ARGS "${LIT_ARGS_DEFAULT}" CACHE STRING "Default options for lit")
   endif()
 
   # Required doc configuration

lldb/cmake/modules/LLDBStandalone.cmake

if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   endif()
 
   set(LLVM_TOOLS_BINARY_DIR ${TOOLS_BINARY_DIR} CACHE PATH "Path to llvm/bin")
   set(LLVM_LIBRARY_DIR ${LIBRARY_DIR} CACHE PATH "Path to llvm/lib")
   set(LLVM_MAIN_INCLUDE_DIR ${INCLUDE_DIR} CACHE PATH "Path to llvm/include")
   set(LLVM_DIR ${LLVM_OBJ_ROOT}/cmake/modules/CMakeFiles CACHE PATH "Path to LLVM build tree CMake files")
   set(LLVM_BINARY_DIR ${LLVM_OBJ_ROOT} CACHE PATH "Path to LLVM build tree")
   set(LLVM_MAIN_SRC_DIR ${MAIN_SRC_DIR} CACHE PATH "Path to LLVM source tree")
-  set(LLVM_EXTERNAL_LIT ${LLVM_TOOLS_BINARY_DIR}/llvm-lit CACHE PATH "Path to llvm-lit")
+  set(LLVM_DEFAULT_EXTERNAL_LIT ${LLVM_TOOLS_BINARY_DIR}/llvm-lit CACHE PATH "Path to llvm-lit")
 
   find_program(LLVM_TABLEGEN_EXE "llvm-tblgen" ${LLVM_TOOLS_BINARY_DIR}
     NO_DEFAULT_PATH)
 
   set(LLVMCONFIG_FILE "${LLVM_CMAKE_PATH}/LLVMConfig.cmake")
   if(EXISTS ${LLVMCONFIG_FILE})
     file(TO_CMAKE_PATH "${LLVM_CMAKE_PATH}" LLVM_CMAKE_PATH)
     list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_PATH}")

llvm/cmake/modules/AddLLVM.cmake

foreach (_variableName ${_variableNames})
     message(STATUS "${_variableName}=${${_variableName}}")
   endforeach()
 endfunction()
 
 function(get_llvm_lit_path base_dir file_name)
   cmake_parse_arguments(ARG "ALLOW_EXTERNAL" "" "" ${ARGN})
 
   if (ARG_ALLOW_EXTERNAL)
-    set(LLVM_DEFAULT_EXTERNAL_LIT "${LLVM_EXTERNAL_LIT}")
     set (LLVM_EXTERNAL_LIT "" CACHE STRING "Command used to spawn lit")
     if ("${LLVM_EXTERNAL_LIT}" STREQUAL "")
       set(LLVM_EXTERNAL_LIT "${LLVM_DEFAULT_EXTERNAL_LIT}")
     endif()
 
     if (NOT "${LLVM_EXTERNAL_LIT}" STREQUAL "")
       if (EXISTS ${LLVM_EXTERNAL_LIT})
         get_filename_component(LIT_FILE_NAME ${LLVM_EXTERNAL_LIT} NAME)