[flang] Use clang_target_link_libraries() for clang dependency (llvm#126037)

This dependency is part of libclang-cpp, so it should use
clang_target_link_libraries.

(cherry picked from commit 7c695e4)

Author: nikic

flang/unittests/Frontend/CMakeLists.txt

@@ -12,7 +12,6 @@ add_flang_unittest(FlangFrontendTests
 
 target_link_libraries(FlangFrontendTests
   PRIVATE
-  clangBasic
   flangFrontend
   flangFrontendTool
   FortranLower
@@ -22,6 +21,11 @@ target_link_libraries(FlangFrontendTests
   FortranEvaluate
 )
 
+clang_target_link_libraries(FlangFrontendTests
+  PRIVATE
+  clangBasic
+)
+
 mlir_target_link_libraries(FlangFrontendTests
   PRIVATE
   MLIRIR