Move floating point related entities to namespace level

Enumerations that describe rounding mode and exception behavior were
defined inside ConstrainedFPIntrinsic. It makes sense to use the same
definitions to represent the same properties in other cases, not only
in constrained intrinsics. It was however inconvenient as required to
include constrained intrinsics definitions even if they were not needed.
Also using long scope prefix reduced readability.

This change moves these definitioins to the namespace llvm::fp.
No functional changes.

Differential Revision: https://reviews.llvm.org/D69552