[IR] Remove support for float binop constant expressions

As part of https://discourse.llvm.org/t/rfc-remove-most-constant-expressions/63179,
this removes support for the floating-point binop constant expressions
fadd, fsub, fmul, fdiv and frem.

As part of this change, the C APIs LLVMConstFAdd, LLVMConstFSub,
LLVMConstFMul, LLVMConstFDiv and LLVMConstFRem are removed.
The LLVMBuild APIs should be used instead.

Differential Revision: https://reviews.llvm.org/D129478