[flang] Add -ffast-math and -Ofast

clang -cc1 accepts -Ofast. I did not add it to flang -fc1 because this
seems redundant because the compiler driver will always resolve -Ofast
into -O3 -ffast-math (I added a test for this).

-menable-infs is removed from the frontend-forwarding test because if
all of the fast-math component flags are present, these will be resolved
into the fast-math flag. Instead -menable-infs is tested in the
fast-math test.

Specifying -ffast-math to the compiler driver causes linker invocations
to include crtfastmath.o.

RFC: https://discourse.llvm.org/t/rfc-the-meaning-of-ofast/66554

Differential Revision: https://reviews.llvm.org/D138675