[LLVM-C] Add accessors to the default floating-point metadata node

Summary: Add a getter and setter pair for floating-point accuracy metadata.

Reviewers: whitequark, deadalnix

Reviewed By: whitequark

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60527

llvm-svn: 358883