[flang] Add -f[no-]associative-math and -mreassociate

Only add the option processing and store the result. No attributes are
added to FIR yet.

Clang only forwards -mreassociate
if (AssociativeMath && !SignedZeros && !TrappingMath)

Flang doesn't have -f[no-]trapping-math, so this part of the condition
has been omitted. !TrappingMath is the default.

Differential Revision: https://reviews.llvm.org/D137329