Refactoring in commit bdc40742 (CMakeDetermineCompilerId: Test without
COMPILER_ID_FLAGS if REQUIRE_SUCCESS, 2021-02-27, v3.20.0-rc3~6^2) added
an extra macro layer through which flag strings are passed.  That caused
an extra level of argument re-parsing, and broke flags with backslashes.
Pass flags to the helper macro through variable names instead.

Fixes: #22041