[LLVM-C] Allow For Creating a BasicBlock without a Parent Function
Summary: Add a utility function for creating a basic block without a parent function. A useful operation for compilers that need to synthesize and conditionally insert code without having to bother with appending and immediately unlinking a block.
Reviewers: whitequark, deadalnix
Reviewed By: whitequark
Differential Revision: https://reviews.llvm.org/D56279