Tests: Decouple COMPILE_LANGUAGE in file(GENERATE) from COMPILE_OPTIONS

Simplify the `RunCMake.File_Generate` test `COMPILE_LANGUAGE-genex`
case to avoid requiring support for the genex in `COMPILE_OPTIONS`.

Tests/RunCMake/File_Generate/COMPILE_LANGUAGE-genex.cmake

@@ -1,12 +1,6 @@
-
 enable_language(CXX C)
 
-add_library(empty empty.cpp empty.c)
-target_compile_options(empty
-  PRIVATE LANG_IS_$<COMPILE_LANGUAGE>
-)
-
 file(GENERATE
   OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/opts-$<COMPILE_LANGUAGE>.txt
-  CONTENT "$<TARGET_PROPERTY:empty,COMPILE_OPTIONS>\n"
+  CONTENT "LANG_IS_$<COMPILE_LANGUAGE>\n"
 )