LLVMBuilder/LLVMFoldingBuilder -> IRBuilder

Hi, the functionality of LLVMFoldingBuilder has been folded
into LLVMBuilder, which has been renamed to IRBuilder. If
you were using LLVMFoldingBuilder then it should be enough
to rename LLVMFoldingBuilder to IRBuilder everywhere (and
change the #include from LLVMBuilder to IRBuilder). If you
were using LLVMBuilder then as well as renaming LLVMBuilder
to IRBuilder you may also need to fix up the types of some
variables since IRBuilder usually returns a Value* rather
than a specific instruction, because it may be returning
an instruction or a folded constant.