[gn build] Port faf4e8a

Author: llvmgnsyncbot

llvm/utils/gn/secondary/clang/unittests/Basic/BUILD.gn

@@ -15,6 +15,7 @@ unittest("BasicTests") {
     "FileEntryTest.cpp",
     "FileManagerTest.cpp",
     "LineOffsetMappingTest.cpp",
+    "OffloadArchTest.cpp",
     "SanitizersTest.cpp",
     "SarifTest.cpp",
     "SourceManagerTest.cpp",