Since commit 13c7bb5b0c (cmGeneratorExpression: Update strip function to
collect parsed expressions, 2025-04-08), the logic to strip generator
expressions from a string made incorrect assumptions about the contents of
generator expressions, leading certain cases to be stripped incorrectly.
Clean up the logic and fix broken behavior, and add test coverage with
`string(GENEX_STRIP)`.
Fixes: #27133