[libc++] Allow overriding the cached value of LIBCXX_TEST_CONFIG

Author: ldionne

libcxx/CMakeLists.txt

@@ -96,7 +96,7 @@ option(LIBCXX_INCLUDE_TESTS "Build the libc++ tests." ${LLVM_INCLUDE_TESTS})
 option(LIBCXX_ENABLE_PARALLEL_ALGORITHMS "Enable the parallel algorithms library. This requires the PSTL to be available." OFF)
 option(LIBCXX_TEST_GDB_PRETTY_PRINTERS "Test gdb pretty printers." OFF)
 set(LIBCXX_TEST_CONFIG "${CMAKE_CURRENT_SOURCE_DIR}/test/configs/legacy.cfg.in" CACHE STRING
-    "The Lit testing configuration to use when running the tests." FORCE) # TODO: Stop using 'FORCE' once we can assume all CMake build dirs have been re-generated
+    "The Lit testing configuration to use when running the tests.")
 set(LIBCXX_TEST_PARAMS "" CACHE STRING
     "A list of parameters to run the Lit test suite with.")