[llvm] [offload] Fix enabling unittests in standalone builds (PR #138418)

[Joseph Huber via llvm-commits]

================
@@ -386,9 +386,12 @@ add_subdirectory(liboffload)
 add_subdirectory(test)
 
 # Add unit tests if GMock/GTest is present
-if (EXISTS ${LLVM_THIRD_PARTY_DIR}/unittest)
-  if (NOT TARGET llvm_gtest)
-    add_subdirectory(${LLVM_THIRD_PARTY_DIR}/unittest ${CMAKE_CURRENT_BINARY_DIR}/third-party/unittest)
-  endif()
+if (NOT LLVM_THIRD_PARTY_DIR)
+  set(LLVM_THIRD_PARTY_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../third-party")
+endif()
+if (EXISTS ${LLVM_THIRD_PARTY_DIR}/unittest AND NOT TARGET llvm_gtest)
+  add_subdirectory(${LLVM_THIRD_PARTY_DIR}/unittest ${CMAKE_CURRENT_BINARY_DIR}/third-party/unittest)
+endif()
+if (TARGET llvm_gtest)
----------------
jhuber6 wrote:

```suggestion
if(NOT LLVM_THIRD_PARTY_DIR)
  set(LLVM_THIRD_PARTY_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../third-party")
endif()
if(EXISTS ${LLVM_THIRD_PARTY_DIR}/unittest AND NOT TARGET llvm_gtest)
  add_subdirectory(${LLVM_THIRD_PARTY_DIR}/unittest ${CMAKE_CURRENT_BINARY_DIR}/third-party/unittest)
endif()
if(TARGET llvm_gtest)
```
CMake around here tends to drop the space after if.

https://github.com/llvm/llvm-project/pull/138418