[FPEnv] Teach the IRBuilder about correct use of the strictfp attribute.

The IRBuilder needs to add the strictfp attribute to function
definitions and calls when constrained floating point is enabled.

Since so far all front ends have had to do is flip the constrained
switch, I've made this patch always add the required attributes
when said constrained switch is enabled. This continues to keep
changes to front ends minimal.

Differential Revision: D69312