[lld] respect LLVM_EXTERNAL_LIT

[kwk]

Summary

Consider a setup without a system-wide installation of lit.
Instead you pass the path to lit like this:

cmake ...
-DLLVM_EXTERNAL_LIT=<PATH_TO_LIT_BINARY> ...

Then you will run into this error:

ninja: error: unknown target 'check-lld'

I have a buildbot builder that fails with this message. Here's the
passage that triggers this error:

https://github.com/llvm/llvm-zorg/blob/d3bfd5ccbceb542098c350e4d071ceceac6854cb/zorg/buildbot/builders/annotated/standalone-build.sh#L194-L239

By using LLVM_EXTERNAL_LIT instead of LLVM_LIT we fix this problem.

See
here for
a description:

Both the LLVM_ROOT and LLVM_EXTERNAL_LIT options are required to do stand-alone builds for all sub-projects. Additional required options for each sub-project can be found in the table below.

[tstellar]

LGTM.

lld/CMakeLists.txt

if(LLVM_INCLUDE_TESTS)
     # Check prebuilt llvm/utils.
     if(EXISTS ${LLVM_TOOLS_BINARY_DIR}/FileCheck${CMAKE_EXECUTABLE_SUFFIX}
         AND EXISTS ${LLVM_TOOLS_BINARY_DIR}/not${CMAKE_EXECUTABLE_SUFFIX})
       set(LLVM_UTILS_PROVIDED ON)
     endif()
 
     if(EXISTS ${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py)
       # Note: path not really used, except for checking if lit was found
-      set(LLVM_LIT ${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py)
+      set(LLVM_EXTERNAL_LIT ${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py)
       if(NOT LLVM_UTILS_PROVIDED)
         add_subdirectory(${LLVM_MAIN_SRC_DIR}/utils/FileCheck utils/FileCheck)
         add_subdirectory(${LLVM_MAIN_SRC_DIR}/utils/not utils/not)
         set(LLVM_UTILS_PROVIDED ON)
         set(LLD_TEST_DEPS FileCheck not)
       endif()
 
       if (NOT TARGET llvm_gtest)
         message(FATAL_ERROR "llvm-gtest not found. Please install llvm-gtest or disable tests with -DLLVM_INCLUDE_TESTS=OFF")
       endif()
     else()
       # Seek installed Lit.
-      find_program(LLVM_LIT
+      find_program(LLVM_EXTERNAL_LIT
                    NAMES llvm-lit lit.py lit
                    PATHS "${LLVM_MAIN_SRC_DIR}/utils/lit"
                    DOC "Path to lit.py")
     endif()
 
-    if(LLVM_LIT)
+    if(LLVM_EXTERNAL_LIT)
       # Define the default arguments to use with 'lit', and an option for the user
       # to override.
       set(LIT_ARGS_DEFAULT "-sv")
       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")