[flang] Add -ffp-contract option processing

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

Only the "off" and "fast" options are supported. "fast-honor-pragmas" is not applicable because we do not implement `#pragma clang fp contract()` in Fortran [1]. "on" is not supported because it is unclear how to fuse only within individual statements. gfortran also does not implement "on": treating it as an "off".

Currently the default value is "off" to preserve existing behavior. gfortran uses "fast" by default and that may be the right thing for flang-new after further discussion in the future, but that can be changed separately. gfortran's documentation is available [[ https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html | here ]].

[1] https://clang.llvm.org/docs/LanguageExtensions.html#extensions-to-specify-floating-point-flags

Reviewed By: vzakhari, awarzynski

Differential Revision: https://reviews.llvm.org/D136080