SourceKit: simplify and modernise CMake for tests

Use `target_compile_definitions` to fix the quoting.  This is particularly
important when running under a non-sh shell (e.g. Windows) where the quoting
rules are different.

Author: compnerd

unittests/SourceKit/SwiftLang/CMakeLists.txt

@@ -1,16 +1,9 @@
 if(NOT SWIFT_HOST_VARIANT MATCHES "${SWIFT_DARWIN_EMBEDDED_VARIANTS}")
-
   add_swift_unittest(SourceKitSwiftLangTests
     CursorInfoTest.cpp
     EditingTest.cpp
     )
-
-  target_link_libraries(SourceKitSwiftLangTests
-    PRIVATE
-    SourceKitSwiftLang
-    )
-
-  set_property(TARGET SourceKitSwiftLangTests APPEND_STRING PROPERTY COMPILE_FLAGS
-    " '-DSWIFTLIB_DIR=\"${SWIFTLIB_DIR}\"'")
-
+  target_link_libraries(SourceKitSwiftLangTests PRIVATE SourceKitSwiftLang)
+  target_compile_definitions(SourceKitSwiftLangTests PRIVATE
+    SWIFTLIB_DIR=\"${SWIFTLIB_DIR}\")
 endif()