[FPEnv][Clang][Driver] Use MarshallingInfoFlag for -fexperimental-strict-floating-point

As of D80952 we are disabling strict floating point on all hosts except
those that are explicitly listed as supported. Use of strict floating point
on other hosts requires use of the -fexperimental-strict-floating-point
flag. This is to avoid bugs like "https://bugs.llvm.org/show_bug.cgi?id=45329"
(which has an incorrect link in the previous review).

In the review for D80952 I was asked to mark the -fexperimental option as
a MarshallingInfoFlag. This patch does exactly that.

Differential Revision: https://reviews.llvm.org/D88987