[FPEnv] Generate constrained FP comparisons from clang

Update the IRBuilder to generate constrained FP comparisons in
CreateFCmp when IsFPConstrained is true, similar to the other
places in the IRBuilder.

Also, add a new CreateFCmpS to emit signaling FP comparisons,
and use it in clang where comparisons are supposed to be signaling
(currently, only when emitting code for the <, <=, >, >= operators).

Note that there is currently no way to add fast-math flags to a
constrained FP comparison, since this is implemented as an intrinsic
call that returns a boolean type, and FMF are only allowed for calls
returning a floating-point type. However, given the discussion around
https://bugs.llvm.org/show_bug.cgi?id=42179, it seems that FCmp itself
really shouldn't have any FMF either, so this is probably OK.

Reviewed by: craig.topper

Differential Revision: https://reviews.llvm.org/D71467