[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

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D56279

llvm-svn: 350608