Clang should probably warn if -fassociative-math is set without -fno-signed-zeros

Clang ignores -fno-signed-zeros unless -fno-signed-zeros is also set, for the sake of matching GCC's behavior.
But IMO silently ignoring options is quite bad UX, **especially** since the Clang docs do not mention this requirement.
Therefore Clang should warn about an ineffective -fassociative-math flag on its command line.

No file or test is named. Start by tracing Clang's handling of -fassociative-math and -fno-signed-zeros, then inspect existing command-line diagnostic tests and the relevant documentation. Done means an ineffective -fassociative-math setting produces a warning and the documented requirement is clear.

Written by the indexing model from the issue text.