[lld] Fixes for unitests in standalone builds (#83670)

1. Replace the obsolete `llvm_add_unittests()` CMake function with an
explicit check for `TARGET llvm_gtest`. This is more consistent with the
rest of the code, and it makes it possible to avoid checking out
`third-party` tree.
2. Add `LLDUnitTests` test dependency to standalone builds. It is
defined unconditionally, and actually necessary to ensure that unit
tests will be built.

lld/CMakeLists.txt

@@ -192,7 +192,9 @@ add_subdirectory(tools/lld)
 
 if (LLVM_INCLUDE_TESTS)
   add_custom_target(LLDUnitTests)
-  llvm_add_unittests(LLD_UNITTESTS_ADDED)
+  if (TARGET llvm_gtest)
+    add_subdirectory(unittests)
+  endif()
   add_subdirectory(test)
 endif()
 

lld/test/CMakeLists.txt

@@ -38,11 +38,10 @@ configure_lit_site_cfg(
   "LLD_SOURCE_DIR"
 )
 
-set(LLD_TEST_DEPS lld)
+set(LLD_TEST_DEPS lld LLDUnitTests)
 if (NOT LLD_BUILT_STANDALONE)
   list(APPEND LLD_TEST_DEPS
     FileCheck
-    LLDUnitTests
     count
     dsymutil
     llc

llvm/cmake/modules/AddLLVM.cmake

@@ -2509,13 +2509,3 @@ function(setup_host_tool tool_name setting_name exe_var_name target_var_name)
     add_custom_target(${target_var_name} DEPENDS ${exe_name})
   endif()
 endfunction()
-
-# Adds the unittests folder if gtest is available.
-function(llvm_add_unittests tests_added)
-  if (EXISTS ${LLVM_THIRD_PARTY_DIR}/unittest/googletest/include/gtest/gtest.h)
-    add_subdirectory(unittests)
-    set(${tests_added} ON PARENT_SCOPE)
-  else()
-    message(WARNING "gtest not found, unittests will not be available")
-  endif()
-endfunction()